Add config key to disable file watcher

[nguiaSoren]

Context: codebase-memory-mcp 0.6.0 has a background watcher that
auto-reindexes on filesystem mtime updates. This is documented behavior,
but there's no documented config key to disable it.

Use case: For projects with frequent doc edits unrelated to code
structure (e.g., audit documentation), every save triggers a reindex
that mutates graph node/edge counts. This makes "graph health"
indicators less useful since baseline counts drift on routine edits.

Suggested config: watcher_enabled = false to fully disable.
Users would then trigger index_repository manually when they want
fresh state.

Workaround currently used: kill the codebase-memory-mcp process
when quiesce is needed; reconnect via /mcp when ready to resume.