Adds MessageRepository to DefaultCoordinationManager for conflict notification:
- Creates info message to agent when merge conflict occurs
- Message includes conflicting file list and resolution instructions
- System-generated messages use senderType='user' with null senderId
- Refactors conflict description into cleaner array.join format
- MergeQueueItem type for merge queue entries
- MergeStatus type: queued | in_progress | merged | conflict
- MergeResult type with task context
- CoordinationManager interface with methods:
- queueMerge: queue completed task for merge
- getNextMergeable: get next task with resolved dependencies
- processMerges: process all ready merges
- handleConflict: bounce-back for conflict resolution
- getQueueState: query queue by status