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/plugin/hooks/create-continuation-hooks.ts

@@ -9,7 +9,7 @@ import {
   createCompactionContextInjector,
   createCompactionTodoPreserverHook,
   createAtlasHook,
-  createAgentHandoffHook,
+  createAgentSwitchHook,
 } from "../../hooks"
 import { safeCreateHook } from "../../shared/safe-create-hook"
 import { createUnstableAgentBabysitter } from "../unstable-agent-babysitter"
@@ -22,7 +22,7 @@ export type ContinuationHooks = {
   unstableAgentBabysitter: ReturnType<typeof createUnstableAgentBabysitter> | null
   backgroundNotificationHook: ReturnType<typeof createBackgroundNotificationHook> | null
   atlasHook: ReturnType<typeof createAtlasHook> | null
-  agentHandoffHook: ReturnType<typeof createAgentHandoffHook> | null
+  agentSwitchHook: ReturnType<typeof createAgentSwitchHook> | null
 }
 
 type SessionRecovery = {
@@ -118,8 +118,8 @@ export function createContinuationHooks(args: {
         }))
     : null
 
-  const agentHandoffHook = isHookEnabled("agent-handoff")
-    ? safeHook("agent-handoff", () => createAgentHandoffHook(ctx))
+  const agentSwitchHook = isHookEnabled("agent-switch")
+    ? safeHook("agent-switch", () => createAgentSwitchHook(ctx))
     : null
 
   return {
@@ -130,6 +130,6 @@ export function createContinuationHooks(args: {
     unstableAgentBabysitter,
     backgroundNotificationHook,
     atlasHook,
-    agentHandoffHook,
+    agentSwitchHook,
   }
 }