feat(cli): add version, indexPath, lastIndexed to status --json (#329)

Adds `version`, `indexPath`, and an ISO `lastIndexed` to `codegraph status --json`, plus a `CodeGraph.getLastIndexedAt()` library method. `agentCount` dropped (no clear consumer). Reworked from contributor PRs #333 and #480.

Co-Authored-By: Javier Gómez <199902626+12122J@users.noreply.github.com>
Co-Authored-By: Ran <8403607+eddieran@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/index.ts

@@ -576,6 +576,15 @@ export class CodeGraph {
     return this.orchestrator.getChangedFiles();
   }
 
+  /**
+   * Most recent index timestamp (ms since epoch) across all tracked files, or
+   * null when nothing is indexed yet. Lets library consumers check index
+   * freshness without shelling out to `codegraph status --json`. (#329)
+   */
+  getLastIndexedAt(): number | null {
+    return this.queries.getLastIndexedAt();
+  }
+
   /**
    * Extract nodes and edges from source code (without storing)
    */