feat(cli): codegraph stop / list to manage background daemons (#861)

Adds first-class daemon control (the #845 pain point: no clean way to stop a
runaway daemon). `codegraph stop [path]` stops the current/given project's
daemon (SIGTERM -> SIGKILL fallback, sweeps artifacts); `stop --all` stops every
daemon; `list`/`ps` shows running daemons (--json for scripts).

Discovery via a small self-healing registry: each daemon records its root under
~/.codegraph/daemons/ on start, removes it on graceful shutdown; readers prune
dead pids. Cross-platform by construction (files + process.kill). Validated live
on macOS, Linux (docker), and Windows (VM): registry unit 6/6 and real-daemon
stop/list 6/6 on each.

src/mcp/daemon.ts

@@ -54,6 +54,7 @@ import {
   getDaemonSocketPath,
 } from './daemon-paths';
 import { CodeGraphPackageVersion } from './version';
+import { registerDaemon, deregisterDaemon } from './daemon-registry';
 
 /** Default idle linger after the last client disconnects. */
 const DEFAULT_IDLE_TIMEOUT_MS = 300_000;
@@ -191,6 +192,10 @@ export class Daemon {
       startedAt: Date.now(),
     };
 
+    // Drop a discovery record so `codegraph list` / `stop --all` can find us.
+    // Best-effort; a missing record only means list's liveness prune covers it.
+    registerDaemon({ root: this.projectRoot, ...lock });
+
     process.stderr.write(
       `[CodeGraph daemon] Listening on ${this.socketPath} (pid ${process.pid}, v${CodeGraphPackageVersion}). Idle timeout ${this.idleTimeoutMs}ms.\n`
     );
@@ -244,6 +249,7 @@ export class Daemon {
     }
     this.engine.stop();
     this.cleanupLockfile();
+    deregisterDaemon(this.projectRoot);
     if (process.platform !== 'win32') {
       try { fs.unlinkSync(this.socketPath); } catch { /* may already be gone */ }
     }