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/config/schema/hooks.ts

@@ -55,7 +55,7 @@ export const HookNameSchema = z.enum([
   "todo-description-override",
   "webfetch-redirect-guard",
   "legacy-plugin-toast",
-  "agent-handoff",
+  "agent-switch",
 ])
 
 export type HookName = z.infer<typeof HookNameSchema>