fix(worker): stateless AbortPlanningMerge after worker restart mid-merge

PlanningMergeOrchestrator._states is in-memory. A worker restart during a
conflict pause left the list repo mid-merge with no recovery path: both
ContinuePlanningMerge and AbortPlanningMerge threw "no in-progress merge",
and re-Approving failed on the IsMidMergeAsync guard.

AbortAsync now falls through to a stateless path when no _states entry exists:
it looks up the parent's list WorkingDir and, if the repo is mid-merge, runs
git merge --abort there directly, then broadcasts PlanningMergeAborted.
Parent remains WaitingForReview — the next Approve restarts the unit merge
(already-Merged child worktrees are skipped as before).

ContinueAsync error message now points to AbortPlanningMerge as the recovery
action. StartAsync mid-merge guard also carries an actionable hint.

Tests: AbortAsync stateless + mid-merge (restart recovery), AbortAsync
stateless + clean repo (clear error).

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

src/ClaudeDo.Worker/Planning/PlanningMergeOrchestrator.cs

@@ -85,7 +85,8 @@ public sealed class PlanningMergeOrchestrator
         }
 
         if (await _git.IsMidMergeAsync(workingDir, ct))
-            throw new InvalidOperationException("repo is mid-merge");
+            throw new InvalidOperationException(
+                "repo is mid-merge; use AbortPlanningMerge to reset the repository, then Approve again");
         if (await _git.HasChangesAsync(workingDir, ct))
             throw new InvalidOperationException("working tree has uncommitted changes");
 
@@ -110,7 +111,8 @@ public sealed class PlanningMergeOrchestrator
     public async Task ContinueAsync(string planningTaskId, CancellationToken ct)
     {
         if (!_states.TryGetValue(planningTaskId, out var state) || state.CurrentSubtaskId is null)
-            throw new InvalidOperationException("no in-progress merge to continue");
+            throw new InvalidOperationException(
+                "no in-progress merge to continue; if the worker was restarted during a conflict, use AbortPlanningMerge to reset the repository");
 
         var current = state.CurrentSubtaskId;
         var result = await _merge.ContinueMergeAsync(current, ct);
@@ -140,13 +142,40 @@ public sealed class PlanningMergeOrchestrator
     public async Task AbortAsync(string planningTaskId, CancellationToken ct)
     {
         if (!_states.TryGetValue(planningTaskId, out var state) || state.CurrentSubtaskId is null)
-            throw new InvalidOperationException("no in-progress merge to abort");
+        {
+            // No in-memory state — worker may have been restarted while a conflict was paused.
+            // Check whether the list repo is still mid-merge and abort it directly.
+            await AbortStatelessAsync(planningTaskId, ct);
+            return;
+        }
 
         await _merge.AbortMergeAsync(state.CurrentSubtaskId, ct);
         _states.TryRemove(planningTaskId, out _);
         await _broadcaster.PlanningMergeAborted(planningTaskId);
     }
 
+    private async Task AbortStatelessAsync(string planningTaskId, CancellationToken ct)
+    {
+        string? workingDir;
+        await using (var ctx = _dbFactory.CreateDbContext())
+        {
+            workingDir = await ctx.Tasks
+                .Where(t => t.Id == planningTaskId)
+                .Select(t => t.List.WorkingDir)
+                .FirstOrDefaultAsync(ct);
+        }
+
+        if (string.IsNullOrWhiteSpace(workingDir) || !await _git.IsMidMergeAsync(workingDir, ct))
+            throw new InvalidOperationException("no in-progress merge to abort");
+
+        await _git.MergeAbortAsync(workingDir, ct);
+        _logger.LogInformation(
+            "Stateless abort of mid-merge for planning task {ParentId} (post-restart recovery)",
+            planningTaskId);
+        await _broadcaster.PlanningMergeAborted(planningTaskId);
+        // Parent remains WaitingForReview — Approve will restart the unit merge from scratch.
+    }
+
     private async Task DrainAsync(string planningTaskId, CancellationToken ct)
     {
         if (!_states.TryGetValue(planningTaskId, out var state)) return;