watch: documented kill switches (--sentinel-file and .squad/ralph-stop) are never read - a running loop cannot be stopped except by signal #1711

Summary

squad watch documents two ways to stop a running loop. Neither is implemented. The only way to stop a watch run today is SIGINT/SIGTERM.

Evidence

--sentinel-file is parsed and threaded, but never read.

• packages/squad-cli/src/cli-entry.ts:641-643 parses the flag.
• packages/squad-cli/src/cli-entry.ts:704 passes it into the watch options.
• packages/squad-cli/src/cli/commands/watch/config.ts:42 declares it, with the doc comment "Path to a sentinel file - watch shuts down gracefully when removed."
• packages/squad-cli/src/cli/commands/watch/config.ts:103,144,155 merge it from CLI and file config.

There is no existsSync / storage.read check against sentinelFile anywhere in watch/index.ts or any capability. The value is resolved into config and then dropped.

.squad/ralph-stop has zero code references.

README.md:277                              touch .squad/ralph-stop
docs/features/watch-next-gen.md:174        -> Set .squad/ralph-stop file
docs/features/watch-next-gen.md:281        touch .squad/ralph-stop
docs/features/watch-next-gen.md:299        rm .squad/ralph-stop  # clear sentinel if present
docs/features/watch-next-gen.md:565        # - .squad/ralph-stop sentinel exists

All five hits are documentation. No source file in packages/ references the string.

Impact

• A user following the README to stop an overnight run has no effect on the process.
• There is no way to stop a loop from outside the process (CI job, scheduled task, another terminal, a remote box) without finding and signalling the PID.
• watch already tracks child PIDs (watch/pid-tracker.ts) and has a clean shutdown() path (watch/index.ts:1026-1041), so the graceful-stop machinery exists - only the trigger is missing.

Proposal

Check for a stop signal at the top of executeRound() and before each interval tick:

1. If config.sentinelFile is set and the file no longer exists, shut down gracefully (matching the documented "shuts down gracefully when removed" semantics).
2. Independently, if <squadDir>/ralph-stop exists, shut down gracefully (matching the documented touch .squad/ralph-stop semantics).
3. Log which trigger fired, finish the in-flight round rather than killing it mid-agent, then run the existing shutdown().

Either implement both, or remove them from the docs and the CLI surface so the documented behaviour matches reality.

Happy to open a PR for this - it is contained to watch/index.ts plus tests.