feat(main): trigger-watcher — file-based input injection for harness scripts

[JeanBaptisteRenard]

Summary

Adds a file-based trigger mechanism so harness scripts can inject keyboard input into any open Switchboard PTY session without requiring Electron IPC.

• trigger-watcher.js — new module (exports start(ctx)). Watches ~/.switchboard/triggers/ for *.json files, validates shape, optionally waits for the session to go idle, writes command + '\r' into the PTY, and leaves a result file in processed/.
• main.js — wires trigger-watcher.start() right after startScheduler, passing closures over activeSessions to expose getPtyForSession and isSessionBusy.
• test/trigger-watcher.test.js — 6 node:test cases: happy path, unknown session, malformed JSON, missing field, idle-wait flip, idle-timeout.
• .ai/contexts/trigger-watcher.md — context doc for future agents.
• ~/.skaleet-ai/conventions/how-to/switchboard-trigger.md — harness-author convention doc (outside repo, not in this PR diff).

Contract

Drop a file at ~/.switchboard/triggers/<uuid>.json:

{ "sessionId": "abc-123", "command": "/compact", "wait": "idle" }

Result appears at ~/.switchboard/triggers/processed/<uuid>.result.json:

{ "ok": true, "sessionId": "abc-123", "command": "/compact", "sent_at": "...", "waited_ms": 320 }

Bash one-liner

UUID=$(uuidgen) && \
  echo '{"sessionId":"'"$SID"'","command":"/compact","wait":"idle"}' > ~/.switchboard/triggers/${UUID}.json && \
  sleep 0.5 && cat ~/.switchboard/triggers/processed/${UUID}.result.json

Full switchboard_send bash function in ~/.skaleet-ai/conventions/how-to/switchboard-trigger.md.

Context doc

.ai/contexts/trigger-watcher.md — covers public surface, invariants, non-obvious behaviors (OSC-title busy detection, inFlight dedup), and the change-also checklist.

Test plan

• task check (lint + tests): 70 pass / 0 fail, 0 new lint errors
• Happy path: trigger → PTY write → result ok:true
• Unknown session: result ok:false, no PTY write
• Malformed JSON: result ok:false, no PTY write
• Missing command field: result ok:false
• wait:"idle" — busy flips to idle after 150 ms: write happens, waited_ms >= 100
• wait:"idle" timeout (200 ms): result ok:false, no PTY write