Root cause fix for issue #927:
- After /plan → /start-work → interruption, in-memory sessionAgentMap is cleared
- getAgentFromMessageFiles() returns 'prometheus' (oldest message from /plan)
- But boulder.json has agent: 'atlas' (set by /start-work)
Fix: Check boulder state agent BEFORE falling back to message files
Priority: in-memory → boulder state → message files
Test: 3 new tests covering the priority logic