feat(worker): use the list base branch as worktree base and default merge target

src/ClaudeDo.Worker/Runner/WorktreeManager.cs

@@ -274,6 +274,13 @@ public sealed class WorktreeManager
                     return parentHead;
             }
         }
-        return await _git.RevParseHeadAsync(workingDir, ct);
+        using var cfgCtx = _dbFactory.CreateDbContext();
+        var baseBranch = await cfgCtx.ListConfigs.AsNoTracking()
+            .Where(c => c.ListId == task.ListId)
+            .Select(c => c.BaseBranch)
+            .FirstOrDefaultAsync(ct);
+        return string.IsNullOrWhiteSpace(baseBranch)
+            ? await _git.RevParseHeadAsync(workingDir, ct)
+            : await _git.RevParseAsync(workingDir, baseBranch, ct);
     }
 }