fix(tasks): harden loop reconciliation against renames and malformed files

Review fixes for the markdown loop feature:

- Loop-owned tasks now adopt by loop file path, not task name, so renaming
  a loop (frontmatter name or UI rename) renames the task in place instead
  of leaving a stale duplicate that keeps running the old definition;
  orphan duplicates of the same file are unscheduled.
- Unparseable loop files are reported to the scheduler as
  definition:null entries: a task whose file still exists is kept with its
  last good definition, and only a genuinely removed file unschedules it.
  Transiently malformed files (mid-edit, bad merge) no longer delete tasks
  or their runtime state.
- Adoption preserves UI-only execution fields (goalEnabled, goalTokenBudget,
  permissionAutoAccept, variant) that the portable format does not define.
- DELETE on a loop-sourced task now returns 400 with guidance to remove the
  loop file, instead of being silently undone by the next reconcile.
- Loops default to enabled: false; discovery of repository content never
  auto-executes scheduled sessions unless the file explicitly enables them.

Regression tests for each fix; DOCUMENTATION.md updated.

packages/web/server/lib/scheduled-tasks/service.js

@@ -78,6 +78,17 @@ export const createScheduledTaskService = (dependencies) => {
     await findProjectByID(projectID);
     const normalizedTaskID = asNonEmptyString(taskID);
     if (!normalizedTaskID) throw new OpenChamberControlError('taskId is required', 400);
+    const current = await projectConfigRuntime.listScheduledTasks(projectID);
+    const existing = current.find((task) => task.id === normalizedTaskID) || null;
+    if (existing?.loopFile) {
+      // Loop tasks are owned by their `.agents/loops` markdown file: deleting
+      // the JSON row would be silently undone by the next reconcile. The file
+      // itself is the removal surface.
+      throw new OpenChamberControlError(
+        'Loop task is managed by its .agents/loops markdown file; delete the file to remove the task',
+        400,
+      );
+    }
     const result = await projectConfigRuntime.deleteScheduledTask(projectID, normalizedTaskID);
     if (!result.deleted) throw new OpenChamberControlError('Task not found', 404);
     await scheduledTasksRuntime.syncProject(projectID);