feat(12-01): extend AgentMode with 'decompose'

- Add 'decompose' to AgentMode union type
- Update agents table mode column enum in database schema
- Update test-helpers.ts CREATE_TABLES_SQL with CHECK constraint
- Add missing getNextNumber implementation (blocking fix)

src/agent/types.ts

@@ -13,8 +13,9 @@ export type AgentStatus = 'idle' | 'running' | 'waiting_for_input' | 'stopped' |
  * - execute: Standard task execution (default)
  * - discuss: Gather context through questions, output decisions
  * - breakdown: Decompose initiative into phases
+ * - decompose: Decompose phase into individual tasks
  */
-export type AgentMode = 'execute' | 'discuss' | 'breakdown';
+export type AgentMode = 'execute' | 'discuss' | 'breakdown' | 'decompose';
 
 /**
  * Options for spawning a new agent