fix(mcp): prevent concurrent background sync

[zc277584121]

Summary

• Add a cross-process sync lock around MCP background sync
• Keep existing background sync behavior while preventing duplicate reindex work across local MCP processes
• Add a multi-process E2E script covering contention, held locks, stale locks, and empty snapshots

Testing

• pnpm --filter @zilliz/claude-context-mcp build
• node packages/mcp/scripts/sync-lock-e2e.mjs
• node packages/mcp/scripts/sync-lock-e2e.mjs --many
• node packages/mcp/scripts/sync-lock-e2e.mjs --held
• node packages/mcp/scripts/sync-lock-e2e.mjs --stale
• node packages/mcp/scripts/sync-lock-e2e.mjs --empty
• git diff --check -- packages/mcp/src/sync.ts packages/mcp/scripts/sync-lock-e2e.mjs