fix(mcp): clean up orphan merkle snapshots

[Showiix]

Summary

• delete the local FileSynchronizer merkle snapshot when cloud sync removes a local codebase that no longer exists in cloud collections
• keep the cleanup best-effort so a local cache deletion failure does not abort cloud snapshot reconciliation
• reuse the existing FileSynchronizer.deleteSnapshot() helper and avoid dropping any collection from cloud sync

Tests

• pnpm --filter @zilliz/claude-context-mcp typecheck
• pnpm --filter @zilliz/claude-context-mcp build
• pnpm --filter @zilliz/claude-context-core typecheck
• pnpm --filter @zilliz/claude-context-core build

Fixes #257