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/drizzle/meta/_journal.json

@@ -246,6 +246,13 @@
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       "tag": "0034_add_task_retry_count",
       "breakpoints": true
+    },
+    {
+      "idx": 35,
+      "version": "6",
+      "when": 1772796561474,
+      "tag": "0035_faulty_human_fly",
+      "breakpoints": true
     }
   ]
 }