refactor(file): yield instance context in watcher

[kitlangton]

Summary

• Yield the file watcher instance context explicitly during state initialization.
• Use the yielded context for watcher roots, protected paths, logs, and git directory lookup.
• Route native watcher callbacks through InstanceState.bind so ALS and Effect-provided contexts both work.

Tests

• bun typecheck
• bun run test test/file/watcher.test.ts