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/src/__tests__/extension.test.ts

@@ -101,6 +101,9 @@ describe("kb pi extension", () => {
         "kb_feature_add",
         "kb_slice_activate",
         "kb_feature_link_task",
+        // Agent tools
+        "kb_agent_stop",
+        "kb_agent_start",
       ];
 
       for (const name of expected) {