refactor(athena): rename session_handoff to switch_agent to avoid confusion with /handoff command

Rename across all layers to eliminate naming ambiguity:
- Tool: session_handoff → switch_agent
- Hook: agent-handoff → agent-switch
- Feature: agent-handoff/ → agent-switch/
- Types: SessionHandoffArgs → SwitchAgentArgs, PendingHandoff → PendingSwitch
- Functions: setPendingHandoff → setPendingSwitch, consumePendingHandoff → consumePendingSwitch

/handoff = inter-session context summary (existing command)
switch_agent = intra-session active agent change (our new tool)

src/tools/index.ts

@@ -38,7 +38,7 @@ export { createCallOmoAgent } from "./call-omo-agent"
 export { createAthenaCouncilTool } from "./athena-council"
 export { createLookAt } from "./look-at"
 export { createDelegateTask } from "./delegate-task"
-export { createSessionHandoffTool } from "./session-handoff"
+export { createSwitchAgentTool } from "./switch-agent"
 export {
   createTaskCreateTool,
   createTaskGetTool,