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/features/agent-switch/state.test.ts

@@ -0,0 +1,50 @@
+import { describe, test, expect, beforeEach } from "bun:test"
+import { setPendingSwitch, consumePendingSwitch, _resetForTesting } from "./state"
+
+describe("agent-switch state", () => {
+  beforeEach(() => {
+    _resetForTesting()
+  })
+
+  //#given a pending switch is set
+  //#when consumePendingSwitch is called
+  //#then it returns the switch and removes it
+  test("should store and consume a pending switch", () => {
+    setPendingSwitch("session-1", "atlas", "Fix these findings")
+
+    const entry = consumePendingSwitch("session-1")
+
+    expect(entry).toEqual({ agent: "atlas", context: "Fix these findings" })
+    expect(consumePendingSwitch("session-1")).toBeUndefined()
+  })
+
+  //#given no pending switch exists
+  //#when consumePendingSwitch is called
+  //#then it returns undefined
+  test("should return undefined when no switch is pending", () => {
+    expect(consumePendingSwitch("session-1")).toBeUndefined()
+  })
+
+  //#given a pending switch is set
+  //#when a new switch is set for the same session
+  //#then the latest switch wins
+  test("should overwrite previous switch for same session", () => {
+    setPendingSwitch("session-1", "atlas", "Fix A")
+    setPendingSwitch("session-1", "prometheus", "Plan B")
+
+    const entry = consumePendingSwitch("session-1")
+
+    expect(entry).toEqual({ agent: "prometheus", context: "Plan B" })
+  })
+
+  //#given switches for different sessions
+  //#when consumed separately
+  //#then each session gets its own switch
+  test("should isolate switches by session", () => {
+    setPendingSwitch("session-1", "atlas", "Fix A")
+    setPendingSwitch("session-2", "prometheus", "Plan B")
+
+    expect(consumePendingSwitch("session-1")).toEqual({ agent: "atlas", context: "Fix A" })
+    expect(consumePendingSwitch("session-2")).toEqual({ agent: "prometheus", context: "Plan B" })
+  })
+})