fix(start-work): always hand off to atlas regardless of current session agent (#3155)

/start-work is the "execute this plan with atlas" command.
Previously, if the user's current session agent was sisyphus (or any
non-prometheus/non-atlas agent), the hook would keep that agent instead
of switching to atlas. This caused sisyphus to execute the plan.

Fix: unconditionally select atlas (when registered) or sisyphus (fallback).
The current session agent is irrelevant to /start-work's intent.

Adds regression test: running /start-work from a sisyphus session must
always hand off to atlas when atlas is available.

src/hooks/start-work/start-work-hook.ts

@@ -12,12 +12,10 @@ import {
 } from "../../features/boulder-state"
 import { log } from "../../shared/logger"
 import {
-  getAgentConfigKey,
   getAgentDisplayName,
   getAgentListDisplayName,
 } from "../../shared/agent-display-names"
 import {
-  getSessionAgent,
   isAgentRegistered,
   updateSessionAgent,
 } from "../../features/claude-code-session-state"
@@ -84,18 +82,9 @@ export function createStartWorkHook(ctx: PluginInput) {
     }
 
     log(`[${HOOK_NAME}] Processing start-work command`, { sessionID: input.sessionID })
-    const currentSessionAgent = getSessionAgent(input.sessionID)
-    const currentSessionAgentKey = currentSessionAgent
-      ? getAgentConfigKey(currentSessionAgent)
-      : undefined
-    const activeAgent = currentSessionAgent
-      && currentSessionAgentKey
-      && currentSessionAgentKey !== "prometheus"
-      && currentSessionAgentKey !== "atlas"
-        ? currentSessionAgent
-        : isAgentRegistered("atlas")
-          ? "atlas"
-          : "sisyphus"
+    const activeAgent = isAgentRegistered("atlas")
+      ? "atlas"
+      : "sisyphus"
     const activeAgentDisplayName = activeAgent === "atlas"
       ? getAgentListDisplayName(activeAgent)
       : getAgentDisplayName(activeAgent)