🤖 fix: validate trunkBranch exists before sub-agent workspace creation

When forkWorkspace() fails during task creation, the fallback logic
used parentMeta.name directly as the trunkBranch for git worktree
creation. This fails when the parent's branch doesn't exist locally
(e.g., SSH workspaces, or branches that were never fetched).

The fix validates that trunkBranch exists as a local branch before
using it, falling back to the repository's default trunk branch
(main/master/etc) if not found.

Fixes the cosmetic error:
  fatal: 'git-debounce-5w4e' is not a commit and a branch
  'agent_explore_7eeffc7df6' cannot be created from it

Author: ammar-agent

src/node/services/taskService.ts

@@ -9,6 +9,7 @@ import type { HistoryService } from "@/node/services/historyService";
 import type { PartialService } from "@/node/services/partialService";
 import type { InitStateManager } from "@/node/services/initStateManager";
 import { log } from "@/node/services/log";
+import { detectDefaultTrunkBranch, listLocalBranches } from "@/node/git";
 import { createRuntime } from "@/node/runtime/runtimeFactory";
 import type { InitLogger, WorkspaceCreationResult } from "@/node/runtime/Runtime";
 import { validateWorkspaceName } from "@/common/utils/validation/workspaceValidation";
@@ -483,9 +484,19 @@ export class TaskService {
       initLogger,
     });
 
-    const trunkBranch = forkResult.success
-      ? (forkResult.sourceBranch ?? parentMeta.name)
-      : parentMeta.name;
+    let trunkBranch: string;
+    if (forkResult.success && forkResult.sourceBranch) {
+      trunkBranch = forkResult.sourceBranch;
+    } else {
+      // Fork failed - validate parentMeta.name is a valid local branch
+      const localBranches = await listLocalBranches(parentMeta.projectPath);
+      if (localBranches.includes(parentMeta.name)) {
+        trunkBranch = parentMeta.name;
+      } else {
+        // Parent branch doesn't exist locally, fall back to default trunk
+        trunkBranch = await detectDefaultTrunkBranch(parentMeta.projectPath, localBranches);
+      }
+    }
     const createResult: WorkspaceCreationResult = forkResult.success
       ? { success: true as const, workspacePath: forkResult.workspacePath }
       : await runtime.createWorkspace({