fix: clear session agent on /start-work to allow mode transition from Prometheus

When transitioning from Prometheus (Planner) to Sisyphus via /start-work,
the session agent was not being cleared. This caused prometheus-md-only
hook to continue injecting READ-ONLY constraints into sisyphus_task calls.

- Add clearSessionAgent() call when start-work command is detected
- Add TDD test verifying clearSessionAgent is called with sessionID

src/hooks/start-work/index.ts

@@ -10,6 +10,7 @@ import {
   clearBoulderState,
 } from "../../features/boulder-state"
 import { log } from "../../shared/logger"
+import { clearSessionAgent } from "../../features/claude-code-session-state"
 
 export const HOOK_NAME = "start-work"
 
@@ -70,6 +71,9 @@ export function createStartWorkHook(ctx: PluginInput) {
         sessionID: input.sessionID,
       })
 
+      // Clear previous session agent (e.g., Prometheus) to allow mode transition
+      clearSessionAgent(input.sessionID)
+
       const existingState = readBoulderState(ctx.directory)
       const sessionId = input.sessionID
       const timestamp = new Date().toISOString()