refactor(mcp): async snapshot lock and workspace-scoped state

[defin85]

Problem

Snapshot persistence used sync busy-wait lock backoff, causing high CPU under lock contention. Snapshot state was global, so multiple workspaces/sessions contended on one lock file.

Changes

• Replaced busy-spin lock retry with async retry + jitter.
• Added lock wait/retry telemetry logs.
• Added serialized snapshot save queue and scheduled/coalesced saves.
• Switched default snapshot scope to workspace-specific file (hash of process.cwd()).
• Added best-effort migration from legacy global snapshot into workspace snapshot.
• Updated MCP handlers to await snapshot saves.
• Reduced indexing progress write frequency via coalesced scheduling.

Breaking Changes

• SnapshotManager.saveCodebaseSnapshot() is now async and must be awaited.
• Default snapshot scope changed from global to workspace.

Validation

• packages/mcp/node_modules/.bin/tsc -p packages/mcp/tsconfig.json --noEmit
• pnpm --filter @zilliz/claude-context-mcp build
• bash scripts/build-local-mcp.sh
• pnpm build

[defin85]

Superseded by #274 to align with CONTRIBUTING.md branch naming ().

[defin85]

Superseded by #274 for branch naming compliance with CONTRIBUTING.md (feature/*).