fix: project-scope terminal, file browser, agent execution, and heartbeat monitor

- TerminalService: use per-project instance map instead of destructive singleton
- File browser: thread projectId through API functions, hooks, and components
- Workspace file editor: pass projectId to read/write operations
- Agent heartbeat: guard execution with project scope validation
- Agent runs/stop: validate heartbeatMonitor matches scoped project
- CLI dashboard/serve: pass rootDir to heartbeatMonitor via server options
- Add getRootDir() to HeartbeatMonitor for scope comparison

Co-authored-by: factory-droid[bot] <138933559+factory-droid[bot]@users.noreply.github.com>

packages/engine/src/agent-heartbeat.ts

@@ -225,6 +225,14 @@ export class HeartbeatMonitor {
     return this.isRunning;
   }
 
+  /**
+   * Get the project root directory this monitor is bound to.
+   * Returns undefined when not configured for execution.
+   */
+  getRootDir(): string | undefined {
+    return this.rootDir;
+  }
+
   /**
    * Register an agent for monitoring with optional session context.
    * @param agentId - The agent ID