fix(research-loop-cap): resolve the data root in code so the loop can run

CLAUDE_PLUGIN_DATA is empty in the Bash tool's process env, and the Phase 5
bash snippet is the cap's only caller. resolveLedgerPath() returned null there
and allowTurn() failed closed, so the budget gate denied turn 1 of every real
run: the loop this delivery exists to bound could never spend a turn, and the
pre-registered measurement could not be run at all.

resolveDataRoot() is now the single root for everything the loop writes --
CLAUDE_PLUGIN_DATA when the harness sets it, ~/.claude/voyage when it does
not. Three consumers resolve through it, which is the point: the cap ledger,
the PreToolUse hook's scope-marker lookup, and the command's bash snippets.
A writer and a reader that resolved the root separately are what made the
enforcement hook allow unconditionally in every real run while CLAUDE.md and
docs/architecture.md called it enforcing.

Same root cause, same commit:
- Marker write and remove now share ONE absolute-path guard and one root; the
  write requires a non-empty CLAUDE_CODE_SESSION_ID before composing the path
  (unset, the marker was named `.json`, which no lookup matches and no TTL
  sweep cleans up).
- The per-turn gates resolve VOYAGE_ROOT with a plugin-cache fallback and
  reserve exit 2 for "gate could not run". Interpolating an empty
  ${CLAUDE_PLUGIN_ROOT} ran `node /lib/...` -> exit 1, which the contract read
  as "privacy gate says no" -- an unsatisfiable rewrite loop no query could
  clear.

Two now-unreachable deny branches are removed rather than left as dead safety
claims (allowTurn's no_plugin_data_dir; the hook's uncountable-ledger deny).
The fail-closed stance stays where it is still real: a ledger that cannot be
WRITTEN denies the turn.

Verified end-to-end through the real bash snippets and the real hook with both
variables stripped and HOME sandboxed: marker written under the fallback root,
8 turns spent, 9th denied, hook exits 2, and exits 0 again after removal.
Note: the fallback exit-2 branch fires against the installed v5.9.1 cache,
which predates lib/util/research-loop-cap.mjs -- correct behaviour, and it
clears when the plugin is reinstalled.

Review findings 2670c10a, fbd6d534, 93550dfb.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011vPSXe88qp5aqWUqbDNWoF

tests/commands/trekresearch.test.mjs

@@ -78,6 +78,58 @@ test('trekresearch — Phase 5 loop is gated on effort == high and names both pr
   assert.match(p5, /\$\{CLAUDE_PLUGIN_ROOT\}/, 'shim invocations must use the ${CLAUDE_PLUGIN_ROOT} path form');
 });
 
+// CLAUDE_PLUGIN_DATA and CLAUDE_PLUGIN_ROOT are substituted in this command's
+// TEXT but are EMPTY in the Bash tool's process env. Every snippet below runs
+// in that env, so each needs a resolution that does not depend on it.
+test('trekresearch — the scope-marker snippets resolve a root instead of requiring CLAUDE_PLUGIN_DATA', () => {
+  const p5 = phase5(read());
+  const blocks = [...p5.matchAll(/```bash\n([\s\S]*?)```/g)].map((m) => m[1]);
+  const write = blocks.find((b) => b.includes('trekresearch-loop-scope') && b.includes('printf'));
+  const remove = blocks.find((b) => b.includes('trekresearch-loop-scope') && b.includes('rm -f'));
+  assert.ok(write, 'Phase 5 must carry the scope-marker write snippet');
+  assert.ok(remove, 'Phase 5 must carry the scope-marker removal snippet');
+
+  for (const [name, block] of [['write', write], ['remove', remove]]) {
+    assert.match(
+      block,
+      /\$\{CLAUDE_PLUGIN_DATA:-\$HOME\/\.claude\/voyage\}/,
+      `the ${name} snippet must fall back to the same root research-loop-cap.mjs resolves`,
+    );
+    assert.match(
+      block,
+      /case .* in\s*\n?\s*\/\*\)/,
+      `the ${name} snippet must guard on ONE absolute-path test — write and remove must not disagree on what counts as usable`,
+    );
+  }
+
+  // Unset, ${CLAUDE_CODE_SESSION_ID} composes a marker named `.json`, which no
+  // hook lookup and no TTL sweep ever matches or cleans up.
+  assert.match(
+    write,
+    /-n "\$\{?CLAUDE_CODE_SESSION_ID/,
+    'the write snippet must require a non-empty CLAUDE_CODE_SESSION_ID before composing the marker path',
+  );
+});
+
+test('trekresearch — the per-turn gates separate "gate could not run" from "gate says no"', () => {
+  const p5 = phase5(read());
+  assert.match(
+    p5,
+    /VOYAGE_ROOT/,
+    'the gate snippet must resolve a plugin root rather than interpolating ${CLAUDE_PLUGIN_ROOT} straight into `node`',
+  );
+  assert.match(
+    p5,
+    /exit 2|could not run/i,
+    'an unresolvable gate must be distinguishable from a denial — otherwise every query reads as a privacy violation no rewrite can clear',
+  );
+  assert.match(
+    p5,
+    /plugins\/cache/,
+    'the fallback must name the plugin cache location it searches',
+  );
+});
+
 test('trekresearch — Phase 5 declares the loop bound and all three exits', () => {
   const p5 = phase5(read());
   assert.match(p5, /### Loop bound/, 'Phase 5 must carry a `### Loop bound` sub-heading');