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/switch-agent/tools.ts

@@ -0,0 +1,37 @@
+import { tool, type ToolDefinition } from "@opencode-ai/plugin"
+import { setPendingSwitch } from "../../features/agent-switch"
+import { updateSessionAgent } from "../../features/claude-code-session-state"
+import type { SwitchAgentArgs } from "./types"
+
+const DESCRIPTION =
+  "Switch the active session agent. After calling this tool, the session will transition to the specified agent " +
+  "with the provided context as its starting prompt. Use this to route work to another agent " +
+  "(e.g., Atlas for fixes, Prometheus for planning). The switch executes when the current agent's turn completes."
+
+const ALLOWED_AGENTS = new Set(["atlas", "prometheus", "sisyphus", "hephaestus"])
+
+export function createSwitchAgentTool(): ToolDefinition {
+  return tool({
+    description: DESCRIPTION,
+    args: {
+      agent: tool.schema
+        .string()
+        .describe("Target agent name to switch to (e.g., 'atlas', 'prometheus')"),
+      context: tool.schema
+        .string()
+        .describe("Context message for the target agent — include confirmed findings, the original question, and what action to take"),
+    },
+    async execute(args: SwitchAgentArgs, toolContext) {
+      const agentName = args.agent.toLowerCase()
+
+      if (!ALLOWED_AGENTS.has(agentName)) {
+        return `Invalid switch target: "${args.agent}". Allowed agents: ${[...ALLOWED_AGENTS].join(", ")}`
+      }
+
+      updateSessionAgent(toolContext.sessionID, agentName)
+      setPendingSwitch(toolContext.sessionID, agentName, args.context)
+
+      return `Agent switch queued. Session will switch to ${agentName} when your turn completes.`
+    },
+  })
+}