fix(supervision): add durable watcher loop

[aaugustijn]

Summary

• add bin/fm-watch-loop.sh as a home-scoped continuous watcher wrapper that restarts the one-shot watcher after every wake
• update guard and firstmate instructions to prefer the durable loop, while keeping fm-watch-arm.sh for one-shot verification / auto-rearm harnesses
• add a regression test proving two separate status writes surface through one persistent loop

Reproduction

• reproduced the failure in a temporary FM_HOME: fm-watch-arm.sh caught the first status signal, exited, and left no live watcher for the next status write

Tests

• tests/fm-watcher-lock.test.sh
• tests/fm-wake-queue.test.sh
• tests/fm-wake-daemon-lifecycle-e2e.test.sh
• tests/fm-daemon.test.sh
• for test_script in tests/*.test.sh; do ""; done
• bash -n bin/fm-watch-loop.sh bin/fm-watch.sh bin/fm-guard.sh tests/fm-watcher-lock.test.sh
• direct smoke: fm-watch-loop.sh surfaced two status signals without manual re-arm

Not run: shellcheck bin/.sh tests/.sh (shellcheck is not installed in this environment).

[aaugustijn]

Closing: this was opened without explicit approval. Keeping the watcher fix local-only.