fix(autopilot): enforce the in-session merge-auth preflight in code

[dngioidev]

Closes #316 (p1 bug, epic #183)

Problem

The autopilot merge-authorization requirement was prose in SKILL.md only. An unattended/scheduled run has no live user turn to grant it, so a run would deliver a whole ticket and then silently wedge at the first merge — the observed failure. The harness auto-mode classifier is not observable from code, so the fix is not to detect it: it is a run-start preflight that requires an EXPLICIT authorization to be recorded, and degrades to PR-only (awaiting-human) when it is absent — instead of proceeding into deliveries that stall.

Change (thin: a preflight decision + ledger record + wiring)

• New plugin/scripts/autopilot/preflight.mjs — pure mergeAuthPreflight({ authorized, config }) returns the effective merge mode: auto-merge only when an explicit in-session grant is held AND config doesn't disable it; otherwise pr-only with the exact human-readable notice the orchestrator surfaces. authorized is true only for a genuine live user grant (a value in run.json / config / allowlist / narration passes false). Thin CLI prints the decision.
• ledger.mjs startRun(cwd, opts) — records mergeMode + mergeReason into run.json at run start (auditable, resume-safe). On resume it keeps the original start time but re-runs the (non-file-backed) preflight. No-opt startRun(cwd) is unchanged (back-compat).
• merge.mjs runMerge(..., { mode }) — pr-only carries autoMergeEnabled:false semantics: the merge path parks the ticket awaiting-human rather than attempting a merge that stalls. The auto-merge path is unchanged when authorized.
• autopilot_merge MCP tool — threads mode through so the live merge honors the recorded decision by construction.
• SKILL.md — the preflight section now references the code preflight as the mechanism (previously "a helper would be additive"); driver-scripts list documents preflight.mjs.

Acceptance criteria

• AC.1 — at run start autopilot verifies an explicit merge authorization; absent it degrades to PR-only (awaiting-human) rather than stalling at the first merge. Verified: mergeAuthPreflight returns pr-only when no grant (and when config opts out); startRun records the decision into run.json; preflight CLI exercised both paths (exit 3 pr-only / exit 0 auto-merge).
• AC.2 — test covers grant-present (proceed to auto-merge) and grant-absent (refuse/degrade to PR-only) paths. Verified: new #316 block in tests/autopilot/engine.test.mjs — AC-316.1 (grant → auto-merge; absent/omitted/non-true → pr-only; config opt-out wins; startRun records + resume refreshes) and AC-316.2 (runMerge mode pr-only parks and never calls pr merge/pr view; auto-merge squash-merges on green; mode omitted preserves prior behavior).

Verification

• pnpm verify — 603/603 green (was 603 pre-change; +7 new autopilot: enforce the in-session merge-auth preflight in code #316 tests, 1 existing autopilot: gate the live merge through the tested merge bar #315 pass-through test updated for the new mode arg, net counts steady across added/updated).
• claude plugin validate ./plugin --strict — passes locally.
• Preflight CLI driven end-to-end (both grant states).
• All existing autopilot behavior/tests preserved.

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