feat: auto-reindex on git events (watchdog, hooks, watch)

[VioletCranberry]

Three composable layers that keep the index in sync without manual reindex, all sharing one core (src/cocosearch/auto_reindex.py):

• MCP server watchdog (on by default, stdio): polls git state every autoReindex.intervalSeconds and triggers an incremental reindex on branch/commit drift, reusing the existing _indexing_lock/_active_indexing pair so it coordinates with /api/reindex.
• cocosearch hooks install/uninstall/status: marker-wrapped post-checkout, post-merge, post-commit, post-rewrite hooks in $(git rev-parse --git-common-dir)/hooks. Append-safe (husky/pre-commit/lefthook coexistence), idempotent, worktree-safe.
• cocosearch watch: foreground polling loop calling run_reindex_sync every --interval seconds (default 30). Cheap because flow.update() is already content-hash incremental.

Adds --if-exists and --quiet flags on cocosearch index so hooks stay silent and never auto-create indexes. Adds AutoReindexSection to config schema (enabled/intervalSeconds) plus env var overrides. Dependency extraction auto-reruns on every trigger when the deps table exists.