feat(FN-3777): add create_agent and delete_agent tools

Adds agent creation and deletion tools to the pi extension, with engine-side implementation in `agent-tools.ts`, core store integration for agent lifecycle management, and corresponding tests and documentation updates across the workspace.

Fusion-Task-Id: FN-3777

packages/core/src/__tests__/agent-store.test.ts

@@ -1247,6 +1247,24 @@ describe("AgentStore", () => {
       expect(handler).toHaveBeenCalledOnce();
       expect(handler).toHaveBeenCalledWith(created.id);
     });
+
+    it("blocks delete when checked-out assigned task exists unless force=true", async () => {
+      const taskStore = new TaskStore(rootDir, join(rootDir, ".fusion-global-settings"));
+      await taskStore.init();
+      const linkedStore = new AgentStore({ rootDir, inMemoryDb: true, taskStore });
+      await linkedStore.init();
+
+      const created = await linkedStore.createAgent({ name: "Checked Out", role: "executor" });
+      const task = await taskStore.createTask({ title: "T", description: "D", column: "todo", assignedAgentId: created.id });
+      await taskStore.updateTask(task.id, { checkedOutBy: created.id });
+
+      await expect(linkedStore.deleteAgent(created.id)).rejects.toThrow("holds checkout");
+      await linkedStore.deleteAgent(created.id, { force: true });
+      expect(await taskStore.getTask(task.id)).toEqual(expect.objectContaining({ assignedAgentId: undefined, checkedOutBy: undefined }));
+
+      linkedStore.close();
+      taskStore.close();
+    });
   });
 
   // ── listAgents ────────────────────────────────────────────────────

packages/core/src/agent-store.ts

@@ -1627,13 +1627,27 @@ export class AgentStore extends EventEmitter {
    * @param agentId - The agent ID
    * @throws Error if agent not found
    */
-  async deleteAgent(agentId: string): Promise<void> {
+  async deleteAgent(agentId: string, options?: { force?: boolean; reassignTo?: string }): Promise<void> {
     await this.withLock(agentId, async () => {
       const agent = await this.getAgent(agentId);
       if (!agent) {
         throw new Error(`Agent ${agentId} not found`);
       }
 
+      if (this.taskStore && typeof (this.taskStore as { getTasksByAssignedAgent?: unknown }).getTasksByAssignedAgent === "function") {
+        const assignedTasks = await this.taskStore.getTasksByAssignedAgent(agentId);
+        for (const task of assignedTasks) {
+          if (task.checkedOutBy === agentId && options?.force !== true) {
+            throw new Error(`Agent ${agentId} holds checkout for task ${task.id}; pass force=true to delete`);
+          }
+
+          await this.taskStore.updateTask(task.id, {
+            assignedAgentId: options?.reassignTo ?? null,
+            checkedOutBy: task.checkedOutBy === agentId ? null : undefined,
+          });
+        }
+      }
+
       this.db.prepare("DELETE FROM agents WHERE id = ?").run(agentId);
       this.db.bumpLastModified();