feat(FN-986): add agent stop/start CLI commands and extension tools

- Add `fn agent stop <id>` and `fn agent start <id>` CLI commands for pausing/resuming agents
- Add `kb_agent_stop` and `kb_agent_start` tools to the pi extension for in-session agent control
- Validate state transitions using AGENT_VALID_TRANSITIONS before applying changes
- Export AgentStore from @fusion/core public API
- Add comprehensive tests for agent CLI commands (210 lines)
- Add changeset for @gsxdsm/fusion patch release

packages/cli/skill/fusion/SKILL.md

@@ -25,6 +25,7 @@ Mission → Milestone → Slice → Feature → Task
 - **Task tools** — `kb_task_create`, `kb_task_update`, `kb_task_list`, `kb_task_show`, `kb_task_attach`, `kb_task_pause`, `kb_task_unpause`, `kb_task_retry`, `kb_task_duplicate`, `kb_task_refine`, `kb_task_archive`, `kb_task_unarchive`, `kb_task_delete`, `kb_task_plan`
 - **GitHub tools** — `kb_task_import_github`, `kb_task_import_github_issue`, `kb_task_browse_github_issues`
 - **Mission tools** — `kb_mission_create`, `kb_mission_list`, `kb_mission_show`, `kb_mission_delete`, `kb_milestone_add`, `kb_slice_add`, `kb_feature_add`, `kb_slice_activate`, `kb_feature_link_task`
+- **Agent tools** — `kb_agent_stop`, `kb_agent_start`
 - **Dashboard** — Use `/fn` command to start/stop the dashboard
 
 </essential_principles>

packages/cli/skill/fusion/references/extension-tools.md

@@ -246,6 +246,22 @@ Link a feature to a kb task. Updates feature status to triaged.
 | `featureId` | string | ✓ | Feature ID (e.g., F-001) |
 | `taskId` | string | ✓ | Task ID (e.g., KB-001) |
 
+### kb_agent_stop
+
+Stop (pause) a running agent. Transitions the agent from running/active to paused state.
+
+| Parameter | Type | Required | Description |
+|-----------|------|----------|-------------|
+| `id` | string | ✓ | Agent ID to stop (e.g., agent-abc123) |
+
+### kb_agent_start
+
+Start (resume) a stopped agent. Transitions the agent from paused to active state.
+
+| Parameter | Type | Required | Description |
+|-----------|------|----------|-------------|
+| `id` | string | ✓ | Agent ID to start (e.g., agent-abc123) |
+
 ## Dashboard Command
 
 ### /fn

packages/cli/skill/fusion/references/fusion-capabilities.md

@@ -35,6 +35,8 @@ Triage → Todo → In Progress → In Review → Done → Archived
 | `kb_feature_add` | Add a feature to a slice |
 | `kb_slice_activate` | Activate a pending slice |
 | `kb_feature_link_task` | Link a feature to a task |
+| `kb_agent_stop` | Stop (pause) a running agent |
+| `kb_agent_start` | Start (resume) a stopped agent |
 
 ## CLI Commands (fn)