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/db/queries.ts

@@ -1421,6 +1421,17 @@ export class QueryBuilder {
     return rows.map(rowToFileRecord);
   }
 
+  /**
+   * Most recent index timestamp (ms since epoch) across all tracked files, or
+   * null when nothing is indexed yet. One indexed aggregate, no per-row scan. (#329)
+   */
+  getLastIndexedAt(): number | null {
+    const row = this.db
+      .prepare('SELECT MAX(indexed_at) AS last FROM files')
+      .get() as { last: number | null } | undefined;
+    return row?.last ?? null;
+  }
+
   /**
    * Get files that need re-indexing (hash changed)
    */