fix: rearm watch sidecars

[steipete]

Summary

• re-arm watcher file sources when chat.db, chat.db-wal, or chat.db-shm is replaced or disappears
• add a parent-directory source so WAL/SHM creation after checkpoint rotation refreshes watched paths
• document the directory watch and fallback re-arm behavior

Fixes #126.

Proof

• swift test --filter MessageWatcherTests
• swift test --filter 'MessageWatcherTests|RPCServerTests|RPCBridgeMessageHandlersTests'
• make lint (passes; existing warning-only lint debt remains)
• /Users/steipete/Projects/agent-scripts/skills/autoreview/scripts/autoreview --mode local --parallel-tests "swift test --filter MessageWatcherTests" clean
• make test still fails locally on existing Full Disk Access dependent live Messages DB send-command tests; the new watcher regression passes inside that run

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: fix: rearm watch sidecars This is item 1/1 in the current shard. Shard 0/1.

This placeholder means the worker is alive and reading the current context. I will edit this same comment with the actual review when the claws are done clicking.

Crustacean status: shell secured, claws on keyboard, evidence pebbles being sorted.