fix(cap): move the enforcement boundary to a denial tombstone, not the turn count

allowTurn() appends BEFORE the turn runs, so during granted turn N the
ledger holds N records. The hook denied at `used >= budget`, which blocked
every tool call of the FINAL granted turn: the primitive granted B turns
and the harness permitted B-1. Worse, an exhausted run therefore always
terminated through an exit-2 tool denial instead of the graceful "cap
exhausted" exit at commands/trekresearch.md - and the prose says in as
many words that exit 2 is not exit 1, so the model was pushed out through
the one exit it is told NOT to treat as a cap.

The review recommended denying at `used > budget`. Taken alone that fixes
the count and breaks the hook: once the O_EXCL claim (previous commit)
makes a breached ledger impossible, `granted > budget` can no longer fire,
and the case this hook exists for - the loop consults the gate, is denied,
and issues the tool call anyway - would be allowed. A deny branch that
cannot be reached is a dead security claim, which is the same thing S82
removed two of rather than leave standing.

So the denial itself became a record. allowTurn() appends a tombstone
{runId, exhausted: true} when it denies for budget, and the hook denies on
the tombstone. Both properties now hold at once:

  granted == budget, no tombstone  -> turn B is in flight   -> ALLOW
  tombstone present                -> the gate already said no -> DENY
  granted  >  budget               -> breached, any cause   -> DENY

A tombstone is not a turn: readLedger reports {granted, exhausted}
separately so it can never consume budget. allowTurn short-circuits on an
existing tombstone, so a hammered gate neither re-walks every slot nor
grows the ledger. The tombstone write is best effort on purpose - the
denial is already the correct answer, so a ledger that cannot take the
record must not turn a denial into a grant.

The parallel-boundary test now asserts GRANTED turns rather than raw
ledger lines, because the denied callers legitimately add tombstones.

Review finding 8eb53458ac3efec778094f9f03b09e1cc1077a09 (MINOR).
Operator decision: tombstone over the literal recommended_action.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LuGhWAbWyRFBFeemfhxoVv

docs/architecture.md

@@ -12,7 +12,7 @@ Imported from `CLAUDE.md` via pointer.
 - `lib/stats/event-emit.mjs` — single-source stats event emitter for autonomy-gate transitions and main-merge-gate (v3.4.0)
 - `lib/validators/{brief,research,plan,progress,session-state}-validator.mjs` — schema validators with CLI shims (`node lib/validators/X.mjs --json <path>`)
 - `lib/validators/architecture-discovery.mjs` — drift-WARN external-contract discovery for `architecture/overview.md`
-- `lib/util/research-loop-cap.mjs` — stateful, **default-off** turn budget for the `/trekresearch` bounded conversation loop. `allowTurn()` derives the used-turn count from its own append-only JSONL ledger; it never asks the caller how many turns it has spent, because a cap that does is not a cap. Each grant first claims a turn **slot** with `O_EXCL` under `trekresearch-loop-claims/`, so the bound survives several callers deciding at once — counting the ledger and then appending is read-then-write, and Phase 4.5/5 can spawn several agents in one message. Budget = `TREKRESEARCH_MAX_CONV_TURNS` (default `3`, invalid values fall back to `3`) × `maxDimensions` (8, `settings.json:16`). Grants 0 unless `VOYAGE_STORM_ENABLED=1`, which is also the second condition on Phase 4.5's skip-guard (that phase does not call this module): unset, **both** STORM phases are inert. `resolveDataRoot()` is the single root for everything the loop writes — `CLAUDE_PLUGIN_DATA` when the harness sets it, `~/.claude/voyage` when it does not (it is empty in the Bash tool's process env, which is where the loop actually runs); the cap hook resolves through the same function, so writer and reader cannot disagree. A ledger that cannot be **written** denies the turn, and one that exists but cannot be **read** denies it too — only `ENOENT` counts as zero turns spent, that being the legitimate first-turn state (fail-closed — the opposite of `event-emit.mjs`, which is telemetry and must never block). The exported `readLedger()` is the single counting rule; the cap hook calls it rather than keeping a private copy. CLI shim: `node lib/util/research-loop-cap.mjs --run-id ID --dimension D --effort E`
+- `lib/util/research-loop-cap.mjs` — stateful, **default-off** turn budget for the `/trekresearch` bounded conversation loop. `allowTurn()` derives the used-turn count from its own append-only JSONL ledger; it never asks the caller how many turns it has spent, because a cap that does is not a cap. Each grant first claims a turn **slot** with `O_EXCL` under `trekresearch-loop-claims/`, so the bound survives several callers deciding at once — counting the ledger and then appending is read-then-write, and Phase 4.5/5 can spawn several agents in one message. Budget = `TREKRESEARCH_MAX_CONV_TURNS` (default `3`, invalid values fall back to `3`) × `maxDimensions` (8, `settings.json:16`). **Default-off:** grants 0 unless `VOYAGE_STORM_ENABLED=1`, which is also the second condition on Phase 4.5's skip-guard (that phase does not call this module): unset, **both** STORM phases are inert. `resolveDataRoot()` is the single root for everything the loop writes — `CLAUDE_PLUGIN_DATA` when the harness sets it, `~/.claude/voyage` when it does not (it is empty in the Bash tool's process env, which is where the loop actually runs); the cap hook resolves through the same function, so writer and reader cannot disagree. A ledger that cannot be **written** denies the turn, and one that exists but cannot be **read** denies it too — only `ENOENT` counts as zero turns spent, that being the legitimate first-turn state (fail-closed — the opposite of `event-emit.mjs`, which is telemetry and must never block). The exported `readLedger()` is the single counting rule; the cap hook calls it rather than keeping a private copy. CLI shim: `node lib/util/research-loop-cap.mjs --run-id ID --dimension D --effort E`
 - `lib/validators/query-privacy-gate.mjs` — gates **every** outbound research query before it leaves the machine; the hard-block tier (secret-shaped strings) is not operator-overridable, so a query that trips it must be reformulated rather than forced through. CLI shim: `node lib/validators/query-privacy-gate.mjs "<query>"`
 
 Wiring points (replaces previous prose-grep instructions):
@@ -33,7 +33,7 @@ Doc-consistency test at `tests/lib/doc-consistency.test.mjs` pins agent-table co
 
 `hooks/scripts/post-bash-stats.mjs` (PostToolUse, CC v2.1.97+) appends `duration_ms` for each Bash call into `${CLAUDE_PLUGIN_DATA}/trekexecute-stats.jsonl`. Useful for finding long-running verify or checkpoint commands.
 
-`hooks/scripts/pre-agent-cap.mjs` (PreToolUse on `WebSearch|WebFetch|Task`) enforces the `/trekresearch` Phase 5 loop bound in the harness, so the cap is not merely prose the model is asked to obey. It counts spent turns read-only from the append-only ledger `research-loop-cap.mjs` writes, and denies (exit 2) once the budget is gone. Scope key = `session_id` + a marker file only the loop writes (`<resolveDataRoot()>/trekresearch-loop-scope/<session_id>.json`, the same root the ledger uses); without a marker the hook allows unconditionally, which is what keeps a globally-wired `PreToolUse` hook from over-blocking unrelated sessions. Stale markers auto-reset on a TTL and `VOYAGE_DISABLE_CAP_HOOK=1` is the kill switch. Defence in depth only — `lib/util/research-loop-cap.mjs` must stay correct if the hook stops firing (see `docs/spike-pretooluse-subagent-reach.md`).
+`hooks/scripts/pre-agent-cap.mjs` (PreToolUse on `WebSearch|WebFetch|Task`) enforces the `/trekresearch` Phase 5 loop bound in the harness, so the cap is not merely prose the model is asked to obey. It counts spent turns read-only from the append-only ledger `research-loop-cap.mjs` writes, through that module's own exported `readLedger()`. It denies (exit 2) once the budget **gate has denied a turn** — the primitive records its denials as tombstones, and the tombstone is the boundary rather than the count, because `allowTurn()` appends before the turn runs and so the final granted turn already shows `budget` records. A ledger showing more granted turns than the budget denies too, as a backstop. Being in scope but unable to count the ledger also denies: a budget control that cannot count must not grant. Scope key = `session_id` + a marker file only the loop writes (`<resolveDataRoot()>/trekresearch-loop-scope/<session_id>.json`, the same root the ledger uses); without a marker the hook allows unconditionally, which is what keeps a globally-wired `PreToolUse` hook from over-blocking unrelated sessions. Stale markers auto-reset on a TTL and `VOYAGE_DISABLE_CAP_HOOK=1` is the kill switch. Defence in depth only — `lib/util/research-loop-cap.mjs` must stay correct if the hook stops firing (see `docs/spike-pretooluse-subagent-reach.md`).
 
 `hooks/scripts/post-compact-flush.mjs` (PostCompact event, v3.4.0) re-injects `.session-state.local.json` after context compaction so multi-session work survives a compaction boundary. Companion to `pre-compact-flush.mjs` (which writes the state file before compaction); together they form the rehydrate cycle that keeps `/trekcontinue` reliable across long-running multi-session work.