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/STANDALONE.md

@@ -103,6 +103,13 @@ fn task import owner/repo           # Import GitHub issues as tasks
 fn task import owner/repo --limit 10 --labels "bug,enhancement"
 ```
 
+### Agent control
+
+```bash
+fn agent stop <agent-id>            # Stop (pause) a running agent
+fn agent start <agent-id>           # Start (resume) a stopped agent
+```
+
 ### Typical workflow
 
 ```bash