Implements acquireLock() in src/runtime/lockfile.ts that writes a PID-stamped
lock file under workspaces_root, detects live vs stale locks via process.kill(pid, 0),
and returns a release function. src/index.ts acquires the lock before DB init and
releases it in the graceful-shutdown handler. Four unit tests cover acquire, duplicate-
acquire rejection, stale-lock recovery, and release.
