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

CLAUDE.md

@@ -46,6 +46,7 @@ Full flag reference for each command (modes, `--gates`, `--profile`, breaking ch
 | plan-critic | opus | Adversarial plan review (9 dimensions) |
 | scope-guardian | opus | Scope alignment (creep + gaps) |
 | session-decomposer | opus | Splits plans into headless sessions with dependency graph |
+| synthesis-agent | opus | Distills Phase-5/7 exploration outputs into a findings digest (NW3 PoC — **dormant**, not wired; delegating Phase 7 alone yields Δ main-context ≈ 0, see `docs/T1-synthesis-poc-results.md`) |
 | docs-researcher | opus | Official documentation, RFCs, vendor docs (Tavily, MS Learn) |
 | community-researcher | opus | Community experience: issues, blogs, discussions |
 | security-researcher | opus | CVEs, audit history, supply chain risks |