fix: track agent in boulder state to fix session continuation (fixes #927)

Add 'agent' field to BoulderState to track which agent (atlas) should
resume on session continuation. Previously, when user typed 'continue'
after interruption, Prometheus (planner) resumed instead of Sisyphus
(executor), causing all delegate_task calls to get READ-ONLY mode.

Changes:
- Add optional 'agent' field to BoulderState interface
- Update createBoulderState() to accept agent parameter
- Set agent='atlas' when /start-work creates boulder.json
- Use stored agent on boulder continuation (defaults to 'atlas')
- Add tests for new agent field functionality

src/hooks/start-work/index.ts

@@ -102,7 +102,7 @@ All ${progress.total} tasks are done. Create a new plan with: /plan "your task"`
             if (existingState) {
               clearBoulderState(ctx.directory)
             }
-            const newState = createBoulderState(matchedPlan, sessionId)
+            const newState = createBoulderState(matchedPlan, sessionId, "atlas")
             writeBoulderState(ctx.directory, newState)
             
             contextInfo = `
@@ -187,7 +187,7 @@ All ${plans.length} plan(s) are complete. Create a new plan with: /plan "your ta
         } else if (incompletePlans.length === 1) {
           const planPath = incompletePlans[0]
           const progress = getPlanProgress(planPath)
-          const newState = createBoulderState(planPath, sessionId)
+          const newState = createBoulderState(planPath, sessionId, "atlas")
           writeBoulderState(ctx.directory, newState)
 
           contextInfo += `