fix: adversarial review fixes — failure shape, CHANGELOG guard, vet coherence

Claude adversarial subagent findings (11), all verified before applying:
- The dispatch JSON contract gains an explicit FAILURE shape
  ({"error":...}) and a parent branch for it — a doc-sync that could not
  run (spawned marking failed, broken preamble) previously had only the
  no-updates shape to emit, which the parent printed as 'Documentation is
  current': a silent false-clean of exactly the VAS-449 class.
- Scope guard now covers CHANGELOG: skip Step 5 voice polish and resolve
  CHANGELOG-touching gates to leave-as-is (the parent authored the
  release entry; the prompt's older auto-choose clause conflicted with
  the contract's never-rewrite invariant).
- Recovery vet is sequence-coherent: pushing a commit pushes its
  ancestors, so ANY non-doc commit (VERSION, package.json, CHANGELOG.md)
  blocks the whole sequence — no more push-the-vetted-child-of-an-
  unvetted-parent hole.
- Steps 7/8/10 failure branches stop a still-running backgrounded task
  before falling back, so a late result never races the inline audit.
- 'Documentation synced' print gated on pushed:true (item 6 owns the
  local-only outcome); doc-review skip note's backward step pointer
  fixed; foreground note scoped 'at initial dispatch' so the sanctioned
  inline FALLBACKS in Steps 7/8 read as sanctioned; constants docstring
  no longer overclaims single-sourcing (names the 3 inline templates).
- Runtime-hook TODO raised P2 → P1: the spawned trust chain is
  agent-self-asserted; prose cannot close it, the hook can.

Both docsync gate E2Es re-verified green on the amended prompt, including
the strict run_in_background === false dispatch assert.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ship/sections/plan-completion.md

@@ -4,7 +4,7 @@
 
 **Dispatch this step as a subagent** using the Agent tool with `subagent_type: "general-purpose"`. The subagent reads the plan file and every referenced code file in its own fresh context. Parent gets only the conclusion.
 
-**Foreground required:** pass `run_in_background: false` on the Agent call — subagents run in the BACKGROUND by default since Claude Code v2.1.198. (Merely omitting the flag no longer produces a foreground run; it must be explicitly false.) Dispatch with the Agent tool itself — never substitute the Skill tool or inline execution to "guarantee" blocking: the explicit flag already makes the Agent call block, and running the work inline forfeits the fresh-context isolation this dispatch exists for. The Gate Logic below consumes this audit's LAST-line JSON before /ship can proceed.
+**Foreground required:** pass `run_in_background: false` on the Agent call — subagents run in the BACKGROUND by default since Claude Code v2.1.198. (Merely omitting the flag no longer produces a foreground run; it must be explicitly false.) Dispatch with the Agent tool itself — never substitute the Skill tool or inline execution at initial dispatch to "guarantee" blocking: the explicit flag already makes the Agent call block, and running the work inline forfeits the fresh-context isolation this dispatch exists for. The Gate Logic below consumes this audit's LAST-line JSON before /ship can proceed.
 
 **Subagent prompt:** Pass these instructions to the subagent:
 
@@ -186,7 +186,7 @@ After producing the completion checklist, evaluate in priority order:
 3. If `deferred > 0` or `unverifiable > 0` and no user override, present the items via the appropriate AskUserQuestion (see Gate Logic priority order above) before continuing.
 4. Embed `summary` in PR body's `## Plan Completion` section (Step 19). If `unverifiable > 0` and the user picked option A in the UNVERIFIABLE gate, also embed `## Plan Completion — Manual Verifications` listing each user-confirmed item.
 
-**If the subagent fails, returns invalid JSON, or never completes (backgrounded despite the flag, or no final output after ~10 minutes — stop waiting):** Fall back to running the audit inline (parent processes the same plan-extraction + classification logic). If the inline fallback also fails (e.g., plan file unreadable, parser error), do NOT silently pass — surface the failure as an explicit AskUserQuestion: "Plan Completion audit could not run ({reason}). Options: (A) Skip audit and ship anyway — record that the audit was skipped in PR body and Step 20 metrics; (B) Stop and fix the audit." Default and recommended option is (B). Silent fail-open is the failure shape that VAS-449 surfaced.
+**If the subagent fails, returns invalid JSON, or never completes (backgrounded despite the flag, or no final output after ~10 minutes — stop waiting; if a backgrounded task is still running, stop it first so a late result never races the fallback):** Fall back to running the audit inline (parent processes the same plan-extraction + classification logic). If the inline fallback also fails (e.g., plan file unreadable, parser error), do NOT silently pass — surface the failure as an explicit AskUserQuestion: "Plan Completion audit could not run ({reason}). Options: (A) Skip audit and ship anyway — record that the audit was skipped in PR body and Step 20 metrics; (B) Stop and fix the audit." Default and recommended option is (B). Silent fail-open is the failure shape that VAS-449 surfaced.
 
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