feat: Add errands table, errand agent mode, and push rollback on merge failure

- Add `errands` table to schema with status enum and relations to agents/projects
- Add `errand` mode to agents.mode enum
- Add push rollback in orchestrator: if push fails after merge, reset to previousRef to preserve the review diff
- Extend MergeResult with previousRef for rollback support; update branch-manager and simple-git-branch-manager
- Add orchestrator tests for push rollback behaviour

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/server/db/schema.ts

@@ -262,7 +262,7 @@ export const agents = sqliteTable('agents', {
   })
     .notNull()
     .default('idle'),
-  mode: text('mode', { enum: ['execute', 'discuss', 'plan', 'detail', 'refine', 'chat'] })
+  mode: text('mode', { enum: ['execute', 'discuss', 'plan', 'detail', 'refine', 'chat', 'errand'] })
     .notNull()
     .default('execute'),
   pid: integer('pid'),
@@ -629,3 +629,35 @@ export const reviewComments = sqliteTable('review_comments', {
 
 export type ReviewComment = InferSelectModel<typeof reviewComments>;
 export type NewReviewComment = InferInsertModel<typeof reviewComments>;
+
+// ============================================================================
+// ERRANDS
+// ============================================================================
+
+export const errands = sqliteTable('errands', {
+  id: text('id').primaryKey(),
+  description: text('description').notNull(),
+  branch: text('branch').notNull(),
+  baseBranch: text('base_branch').notNull().default('main'),
+  agentId: text('agent_id').references(() => agents.id, { onDelete: 'set null' }),
+  projectId: text('project_id').references(() => projects.id, { onDelete: 'cascade' }),
+  status: text('status', {
+    enum: ['active', 'pending_review', 'conflict', 'merged', 'abandoned'],
+  }).notNull().default('active'),
+  createdAt: integer('created_at', { mode: 'timestamp' }).notNull(),
+  updatedAt: integer('updated_at', { mode: 'timestamp' }).notNull(),
+});
+
+export const errandsRelations = relations(errands, ({ one }) => ({
+  agent: one(agents, {
+    fields: [errands.agentId],
+    references: [agents.id],
+  }),
+  project: one(projects, {
+    fields: [errands.projectId],
+    references: [projects.id],
+  }),
+}));
+
+export type Errand = InferSelectModel<typeof errands>;
+export type NewErrand = InferInsertModel<typeof errands>;