feat(voyage): S12 — NW3 synthesis-agent built + measured → declined per measurement [skip-docs]

NW3 (CC-26 §6 PoC): delegate trekplan Phase 7 synthesis to a synthesis-agent,
adopt only if Δ main-context ≥30% with no quality loss. Operator chose the
deterministic-proof path (live ≥3-run bake-off was env-blocked: no API key;
installed plugin is a cache copy so a new agent is invisible to `claude -p`).

Decisive structural finding: trekplan Phase 5 runs the swarm FOREGROUND, so its
outputs are already resident in main before Phase 7. Delegating only Phase 7
evicts nothing → Δ_faithful = 0% (BASE-independent). The ≥30% saving needs an
out-of-scope Phase-5 redesign (swarm-writes-to-disk / nested orchestrator).
VERDICT: DECLINED per measurement.

- agents/synthesis-agent.md — dormant, schema-conformant deliverable (NOT wired)
- lib/plan/synthesis-digest-schema.mjs — digest output contract (+ tests)
- scripts/synthesis-measure.mjs — deterministic Δ-accounting core (+ tests)
- tests/fixtures/synthesis/ — 7 exploration outputs + representative digest
- docs/T1-synthesis-poc-results.md — measurement + verdict (reproducible)
- CLAUDE.md — agent table row (doc-consistency: 24 agents)

Tests 670 → 695 (693 pass / 2 skip / 0 fail). `claude plugin validate` clean
(only the pre-existing root-CLAUDE.md warning). commands/trekplan.md untouched.

[skip-docs] rationale: no user-facing feature ships (NW3 declined; agent dormant
and unwired). The substantive doc is docs/T1-synthesis-poc-results.md; the
README/CHANGELOG roll-up for NW1–NW3 is the S13 coordinated release per
docs/W1-narrow-wins-plan.md §S13.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LqBYc8Ltrk7LipyJmGxXiB

tests/fixtures/synthesis/exploration/dependency-tracer.md

@@ -0,0 +1,76 @@
+# Dependency & Data-Flow Report — output of dependency-tracer
+
+Task: Add a per-wave concurrency cap to trekexecute headless launches.
+
+## Import / call chain relevant to the task
+
+```
+commands/trekexecute.md  (Phase 2.6 prose)
+  └─ generates → templates/headless-launch-template.md  (Bash here-doc)
+        ├─ reads  SHARED_CONTEXT_FILE  (append-system-prompt-file)
+        ├─ spawns claude -p  ×N  (one per wave member, backgrounded with &)
+        └─ waits  via `wait` on collected PIDs
+  └─ consumes → .session-state.local.json  (Handover 7; session graph)
+```
+
+## Data flow
+
+1. session-decomposer emits a plan with wave groupings + a dependency graph.
+2. trekexecute Phase 2.6 turns each independent wave into a launch batch.
+3. The template loops over batch members and backgrounds each `claude -p`,
+   collecting PIDs into a Bash array, then `wait`s for the whole batch.
+4. There is **no slot-limiting** between "background member" and "wait" — the
+   degree of parallelism equals the batch size.
+
+## Side effects
+
+- Each subprocess does `git` work under `GIT_OPTIONAL_LOCKS=0`; high concurrency
+  raises the chance of index-lock contention (mitigated, not eliminated).
+- `--max-budget-usd` is per-subprocess; total spend scales with batch size, so a
+  concurrency cap also indirectly bounds burst spend.
+
+## What a cap touches
+
+- **Pure-addable:** a `maxParallel` resolver in `lib/` (reads plan/profile/flag,
+  returns an integer ≥ 1). No existing module imports would change.
+- **Wire-in:** the template's loop must consume slots (e.g. a counting semaphore
+  in Bash, or `xargs -P <n>`). This is the only behavioral edit.
+
+## No hidden dependents
+
+Grepped for other call sites of the launch template — only trekexecute Phase 2.6
+and the headless-launch-template test reference it. A cap is local in blast
+radius.
+
+## Resolver input provenance (what the cap reads)
+
+The resolved integer must be derived from, in lookup order:
+
+1. **CLI flag** `--max-parallel <n>` — parsed by `lib/parsers/arg-parser.mjs`;
+   highest precedence (operator override).
+2. **Brief signal** — `phase_signals` already carries per-phase orchestration
+   shape; a `max_parallel` hint here is honoured if no flag.
+3. **Profile** — `lib/profiles/` resolves `--profile economy|balanced|premium`;
+   each profile can carry a `max_parallel` default. This is the same lookup
+   order `phase_models` uses, so the resolver should *reuse* profile-resolver,
+   not re-implement precedence.
+4. **Hard default** — `batchSize` (i.e. no cap / current behavior), so the change
+   is a strict no-op until someone opts in.
+
+## Downstream of the cap
+
+- **Budget:** total burst spend = `min(cap, batchSize) × per-member --max-budget-usd`.
+  A cap therefore tightens the worst-case spend envelope — worth noting in the
+  plan's risk/observability section.
+- **Git contention:** fewer concurrent `git`-touching subprocesses → fewer
+  `index.lock` races. `GIT_OPTIONAL_LOCKS=0` reduces lock acquisition but does
+  not serialize ref updates; the cap is the structural mitigation.
+- **Classifier exposure:** a smaller concurrent fan-out under `auto`/`bypass`
+  lowers the surface the proliferation classifier (S7 F4) scrutinises.
+
+## Data-flow invariant to preserve
+
+`SHARED_CONTEXT_FILE` is built once per wave and read by every member; the cap
+must not cause it to be rebuilt per-slot (would defeat the cache prefix). Slot
+limiting happens at spawn time only; the context file is wave-scoped, not
+slot-scoped.