fix(bin): harden watcher continuity bound and snapshot temp cleanup

[keenvc]

Summary

• Continuity supervisor: TERM/INT/HUP cleanup exits after pidfile removal; floor STALE_SECS so a mis-set env cannot recreate the kill loop.
• Watch heartbeat refresh during check sweep so a healthy serial sweep cannot look stuck.
• Fleet snapshot: INT/TERM traps run cleanup; aged /tmp/fm-fleet-tasks.* reap on startup.
• Regression tests for the bound floor, TERM cleanup exit, and snapshot temp reap.

Test plan

• CI green on keenvc/firstmate
• Continuity/snapshot tests in the PR pass