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/git-historian.md

@@ -0,0 +1,31 @@
+# Git History — output of git-historian
+
+Task: Add a per-wave concurrency cap to trekexecute headless launches.
+
+## Recent changes touching the launch path
+
+| Commit (illustrative) | Area | Relevance |
+|-----------------------|------|-----------|
+| Phase 2.6 hardening series | trekexecute.md + headless-launch-template.md | Added `GIT_OPTIONAL_LOCKS`, `--max-budget-usd`, push-before-cleanup, GH#36071 workaround. The launcher is actively maintained and recently hardened — a cap is the next natural hardening. |
+| session-decomposer wiring | agents/session-decomposer.md | Established the wave/dependency graph the launcher consumes. Stable; not the edit site. |
+| profile system (v4.1) | lib/profiles/ | Introduced `phase_models` + `--profile`; the lookup order a `max_parallel` knob should reuse. |
+
+## Ownership / hot files
+
+- `templates/headless-launch-template.md` is a **hot file** — multiple recent
+  hardening commits. Expect a strict doc-consistency needle list; any edit must
+  keep all existing needles AND add the new one.
+- `commands/trekexecute.md` Phase 2.6 is co-edited with the template in every
+  hardening commit ("template mirrors Phase 2.6"). Keep them in lockstep.
+
+## Active branches / risk of conflict
+
+Single active branch (`main`); polyrepo, frequent small commits. Low conflict
+risk. The kjøremodus is one-task-per-session, so this change should be a single
+focused commit touching resolver + template + prose + tests.
+
+## Signal
+
+The project's own history shows a consistent preference: bound risky externalities
+explicitly (budget, locks, turns). An unbounded fan-out is the conspicuous gap in
+that pattern — the change is in-character with how this launcher has evolved.