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/db/repositories/drizzle/test-helpers.ts

@@ -79,7 +79,7 @@ CREATE TABLE IF NOT EXISTS agents (
   session_id TEXT,
   worktree_id TEXT NOT NULL,
   status TEXT NOT NULL DEFAULT 'idle',
-  mode TEXT NOT NULL DEFAULT 'execute',
+  mode TEXT NOT NULL DEFAULT 'execute' CHECK(mode IN ('execute', 'discuss', 'breakdown', 'decompose')),
   created_at INTEGER NOT NULL,
   updated_at INTEGER NOT NULL
 );

src/db/schema.ts

@@ -184,7 +184,7 @@ export const agents = sqliteTable('agents', {
   })
     .notNull()
     .default('idle'),
-  mode: text('mode', { enum: ['execute', 'discuss', 'breakdown'] })
+  mode: text('mode', { enum: ['execute', 'discuss', 'breakdown', 'decompose'] })
     .notNull()
     .default('execute'),
   createdAt: integer('created_at', { mode: 'timestamp' }).notNull(),