fix(mcp): reap serve --mcp child when parent is SIGKILL'd (#286)

Add a PPID watchdog to the MCP server so a `codegraph serve --mcp` child terminates when its host (Claude Code, opencode, …) is force-killed — OOM killer, `kill -9`, container teardown — and the stdin close handlers don't fire. The child would otherwise linger indefinitely, holding inotify watches, file descriptors, and the SQLite WAL.

Also propagates the host PID across the `--liftoff-only` re-exec (CODEGRAPH_HOST_PPID) so the watchdog reaps the orphan on the from-source path too, not just the bundled launcher. Poll interval is CODEGRAPH_PPID_POLL_MS (default 5000ms, 0 disables).

Resolves #277.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>