feat(athena): add session handoff with Question tool for Atlas/Prometheus routing

After Athena synthesizes council findings, presents user with Question tool
TUI to choose: Atlas (fix now), Prometheus (create plan), or no action.
On selection, session_handoff tool stores intent + calls updateSessionAgent(),
then agent-handoff hook fires on session.idle to switch the main session's
active agent via promptAsync with synthesis context.

src/tools/session-handoff/tools.ts

@@ -0,0 +1,37 @@
+import { tool, type ToolDefinition } from "@opencode-ai/plugin"
+import { setPendingHandoff } from "../../features/agent-handoff"
+import { updateSessionAgent } from "../../features/claude-code-session-state"
+import type { SessionHandoffArgs } 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 hand off work to another agent " +
+  "(e.g., Atlas for fixes, Prometheus for planning). The handoff executes when the current agent's turn completes."
+
+const ALLOWED_AGENTS = new Set(["atlas", "prometheus", "sisyphus", "hephaestus"])
+
+export function createSessionHandoffTool(): ToolDefinition {
+  return tool({
+    description: DESCRIPTION,
+    args: {
+      agent: tool.schema
+        .string()
+        .describe("Target agent name to hand off 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: SessionHandoffArgs, toolContext) {
+      const agentName = args.agent.toLowerCase()
+
+      if (!ALLOWED_AGENTS.has(agentName)) {
+        return `Invalid handoff target: "${args.agent}". Allowed agents: ${[...ALLOWED_AGENTS].join(", ")}`
+      }
+
+      updateSessionAgent(toolContext.sessionID, agentName)
+      setPendingHandoff(toolContext.sessionID, agentName, args.context)
+
+      return `Handoff queued. Session will switch to ${agentName} when your turn completes.`
+    },
+  })
+}