fix: harden handoff execution integrity

[stanleyrprose]

Harden long/cross-agent handoff execution around two concrete failure modes observed in a real SignalForge run: stale in-flight receipts after executor termination and unintended remote Git/GitHub mutation before review.

What changes:

• execute-handoff now publishes parent/child PIDs and records non-terminal interrupting while SIGINT/SIGTERM child termination is in progress; terminal interrupted is written only after the child has actually exited.
• child termination no longer treats Node's child.killed flag as proof of process exit; stubborn children that ignore SIGTERM escalate to SIGKILL.
• non-completed terminal runs explicitly separate process state from semantic side-effect outcome with execution_outcome=unknown and reconcile_required=true.
• wait_for_handoff keeps reporting an in-flight state if the recorded child executor is still alive even when the parent is gone; only when both recorded parent and child are gone does it derive orphaned and require reconciliation.
• orphaned receipts do not automatically reuse stale status/diff/log artifacts unless those artifact paths were explicitly recorded by that run.
• local handoff executors default to a lightweight remote-mutation guard: standard git push/send-pack push paths and gh are blocked; common GitHub token env vars are not inherited. --allow-remote-mutations explicitly restores the previous behavior.
• the remote-mutation guard is an accidental-side-effect guard, not a security sandbox; absolute alternate executables/direct network APIs are intentionally outside its guarantee.

Scope intentionally excluded: no persistent PTY/job management, workflow service, DB, queue, new MCP, npm release, or publish.

Verification on macOS from baseline 587f7fd3a4644a847bba13aeb49336056052e1f6, final head 77b56d4512a5aa8cc081db67aa7cb94ce140f0da:

• npm ci — success (registry currently reports 2 moderate + 1 high dependency advisories; not introduced by this change)
• npm run build — PASS
• node scripts/execute-handoff-smoke.mjs — PASS
• node scripts/smoke.mjs — PASS
• npm run smoke — PASS
• npm run stress — PASS
• git diff --check — PASS

New regression coverage includes:

• default git push blocked and explicit opt-in allowed against a local bare remote;
• SIGTERM first produces non-terminal interrupting, then terminal interrupted only after a stubborn child is actually terminated;
• dead parent + live child remains in-flight;
• dead parent + dead child is surfaced as orphaned and reconciliation-required;
• orphaned stale receipts do not expose prior-run artifacts.

[stanleyrprose]

Implementation/review update for exact head 77b56d4512a5aa8cc081db67aa7cb94ce140f0da:

• Spec Fidelity: PASS
• Engineering Standards: PASS after two review fixes: (1) live child executor is never derived as orphaned merely because the parent is gone; (2) SIGINT/SIGTERM now uses non-terminal interrupting until the child actually exits, with stubborn-child SIGKILL escalation based on actual exit/signal state rather than Node's child.killed flag.
• Local verification: npm run build, targeted execute-handoff smoke, MCP smoke, full npm run smoke, npm run stress, and git diff --check all PASS.
• GitHub currently reports 0 check runs for both this upstream fork PR and a CI-only mirror PR in the fork, despite the repository workflow declaring pull_request; CI therefore appears not to have started/been approved yet. No upstream merge or npm publish is being requested automatically.

[stanleyrprose]

Cross-platform CI update for exact head 77b56d4512a5aa8cc081db67aa7cb94ce140f0da:

A CI-only mirror PR in the fork was used because upstream fork checks remained unstarted. After re-registering the fork's existing CI workflow (no code/workflow-content change), run 34510223428 completed successfully on both matrix targets:

• Ubuntu: PASS — Install, Build, Smoke Test, Stress Test, Check Package Contents
• Windows: PASS — Install, Build, Smoke Test, Stress Test, Check Package Contents

The CI-only fork PR has now been closed without merge. Upstream PR #130 remains the canonical integration PR; no upstream merge or npm publish was performed.