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/hooks/agent-switch/hook.ts

@@ -0,0 +1,37 @@
+import type { PluginInput } from "@opencode-ai/plugin"
+import { consumePendingSwitch } from "../../features/agent-switch"
+import { log } from "../../shared/logger"
+
+const HOOK_NAME = "agent-switch" as const
+
+export function createAgentSwitchHook(ctx: PluginInput) {
+  return {
+    event: async (input: { event: { type: string; properties?: Record<string, unknown> } }): Promise<void> => {
+      if (input.event.type !== "session.idle") return
+
+      const props = input.event.properties as Record<string, unknown> | undefined
+      const sessionID = props?.sessionID as string | undefined
+      if (!sessionID) return
+
+      const pending = consumePendingSwitch(sessionID)
+      if (!pending) return
+
+      log(`[${HOOK_NAME}] Switching to ${pending.agent}`, { sessionID })
+
+      try {
+        await ctx.client.session.promptAsync({
+          path: { id: sessionID },
+          body: {
+            agent: pending.agent,
+            parts: [{ type: "text", text: pending.context }],
+          },
+          query: { directory: ctx.directory },
+        })
+
+        log(`[${HOOK_NAME}] Switch to ${pending.agent} complete`, { sessionID })
+      } catch (err) {
+        log(`[${HOOK_NAME}] Switch failed`, { sessionID, error: String(err) })
+      }
+    },
+  }
+}

src/hooks/agent-switch/index.ts

@@ -0,0 +1 @@
+export { createAgentSwitchHook } from "./hook"