feat(FN-1950): add list_agents and delegate_task agent delegation tools

- Add list_agents tool for discovering agents by role, state, or includeEphemeral filter
- Add delegate_task tool for assigning work to a specific agent by ID
- Add comprehensive test coverage for both delegation tools in agent-tools-delegation.test.ts
- Update executor.ts to wire up delegation tools when agentStore is configured
- Document both tools in AGENTS.md and docs/agents.md

docs/agents.md

@@ -296,6 +296,15 @@ Behavior:
   - `maxSpawnedAgentsGlobal` (default 20)
 - Child sessions terminate when parent task ends
 
+## Agent Delegation
+
+Executor and heartbeat agents can discover and delegate work to other agents using two built-in tools:
+
+- **`list_agents`** — List available agents with optional filters (role, state, includeEphemeral)
+- **`delegate_task`** — Create a task and assign it to a specific agent; the task enters `todo` and the agent picks it up on their next heartbeat
+
+Delegation is designed for cross-agent handoff (e.g., an executor handing off to a QA agent). For parallel worktree-based parallelization, use `spawn_agent` instead.
+
 ## Heartbeat Monitoring and Trigger Scheduling
 
 Fusion's `HeartbeatTriggerScheduler` supports five trigger types: