Auto-reindex on cora serve startup (Phase 1) #434

Part of #433.

Task

Auto-reindex stale files when cora serve starts, before accepting MCP connections.

Implementation

1. Add function check_and_reindex_if_stale(conn, project_id) in src/index/mod.rs
2. Query: SELECT path, fingerprint, last_indexed FROM files WHERE project_id = ?
3. For each file, compute current mtime:size fingerprint
4. If changed → re-extract symbols + store
5. Call from src/mcp/server.rs before starting stdin loop

Spec

$ cora serve
[cora] Checking for stale files...
[cora] Auto-indexed 3 changed files (42ms)
[cora] MCP server ready on stdio

Performance

• Stale check: stat() per tracked file — ~0.1ms per 100 files
• Reindex: existing incremental pipeline (~9ms for no changes)
• Must NOT block MCP startup > 1s

Acceptance

• Modify a file → run cora serve → new symbol appears in brain_search
• No stale data after serve starts
• Verbose logging shows what was reindexed

Est: 1 day