feat(trigger-watcher): add chain field for sequential injection

[JeanBaptisteRenard]

Adds a chain field to the trigger JSON for sequential multi-step injection, mutually exclusive with command. Each step waits for the previous turn to complete (busy-rise → busy-fall) before the next command is injected.

Stack note (REBASED 2026-05-31 21:34 CEST): Originally stacked on #26 (timeout default + per-trigger timeout_ms), this branch has been rebased onto main now that #26 (ffbeb44) and #28 (8746466 — liveness check) are merged. The chain loop now also performs the W7 liveness check before each step's write — defense-in-depth across PRs.

Schema

{
  "sessionId": "...",
  "wait": "idle",
  "chain": [
    {"command": "/compact"},
    {"command": "verify and commit"},
    {"command": "open PR"}
  ],
  "timeout_ms": 600000
}

• chain is a non-empty array, length ≤ 20
• Each entry has a required command (same MAX_COMMAND_LEN and forbidden-chars rules as the single-command path)
• Optional per-step timeout_ms overrides the global for that step only (capped at remaining global deadline)
• chain and command are mutually exclusive — both present → ok:false, error:"command and chain are mutually exclusive"

Execution semantics

1. After the initial idle wait (honoring wait field), inject step 0
2. Wait for _cliBusy to flip true (rise, max 2s — TOCTOU window for instant replies), then false (turn complete) before injecting step N+1
3. Global timeout_ms is the deadline for the whole chain. If exceeded mid-chain → abort with {ok:false, partial:true, steps_completed:N, error:"chain timeout"}
4. Per-step timeout_ms bounds only that step's turn-completion wait (independent of remaining global, but capped by it)
5. On any step failure (timeout, session-exit, write error, dead PTY) → stop, write partial result with steps_completed: N (fully-completed step count, not including the failing one)
6. No turn-completion wait after the LAST step — result is written immediately after final injection

Result shape

Success:

{
  "ok": true,
  "sessionId": "...",
  "sent_at": "<step 0 sent_at>",
  "steps": [
    {"idx": 0, "command": "/compact", "sent_at": "...", "waited_ms": 0},
    {"idx": 1, "command": "verify and commit", "sent_at": "...", "waited_ms": 1234}
  ],
  "total_waited_ms": 1234
}

Partial failure: ok:false, partial:true, steps_completed:N, error:"...", steps:[...], total_waited_ms:N

Tests

36 trigger-watcher tests pass: 22 existing + 11 chain (CHAIN-1..CHAIN-11) + 3 W7 liveness (from #28).

The makeChainCtx helper now also stubs pid: process.pid and isPtyAlive: () => true so the new W7 liveness checks (pre-flight + per-step in the chain loop) don't break chain tests.

Out-of-scope follow-ups

• Husky pre-commit hook is not marked executable — chmod +x .husky/pre-commit would re-enable auto-lint+test on commits (separate cleanup PR)
• Taskfile.yaml test task description says "24 tests" but the suite is now at 36+