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/task-finder.md

@@ -0,0 +1,36 @@
+# Task-Relevant Code — output of task-finder
+
+Task: Add a per-wave concurrency cap to trekexecute headless launches.
+
+## Direct hits
+
+| File | Lines | Why relevant |
+|------|-------|--------------|
+| `templates/headless-launch-template.md` | ~10–60 | The Bash fan-out: backgrounds each wave member, `wait`s on PIDs. Edit site for slot-limiting. |
+| `commands/trekexecute.md` | Phase 2.6 | Prose that generates the launch batches; where a `--max-parallel` flag + profile lookup would be documented. |
+| `lib/profiles/` | resolver dir | Profiles already carry per-phase knobs; a `max_parallel` default fits the existing `phase_signals`/profile lookup order. |
+| `lib/util/result.mjs` | 33 | `issue()` — the error shape a `maxParallel` resolver returns on bad input. |
+
+## Reuse candidates
+
+- **arg-parser** (`lib/parsers/arg-parser.mjs`, per tests/lib/arg-parser.test.mjs)
+  already parses `--flag value` pairs for the commands. A `--max-parallel <n>`
+  flag plugs into the existing parser; do not hand-roll parsing.
+- **profile-resolver** (`lib/profiles/`, profile-resolver.test.mjs) is the lookup
+  order CLI-flag → brief signal → profile → default. A concurrency default
+  belongs as a profile field consumed through this same resolver.
+- **autonomy-gate** (`lib/` autonomy-gate.test.mjs) shows the established pattern
+  for "bounded integer with a safe default" — mirror its clamp/validate.
+
+## Existing similar solutions
+
+`scripts/q3-cache-prefix-experiment.mjs` spawns children **sequentially** to
+avoid spawn-burst rate-limits — confirms the project already knows unbounded
+fan-out is a risk and chose a manual bound there. A cap generalises that
+instinct to the headless wave path.
+
+## Models / config
+
+No DB. Config lives in `settings.json` (`trekplan`/`trekresearch` scopes only —
+doc-consistency pins this) and per-run profiles. A `max_parallel` knob should
+ride the profile system, not add a new settings.json scope.