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.test.js

@@ -0,0 +1,65 @@
+import { describe, expect, it, vi } from 'vitest';
+import { createScheduledTaskService } from './service.js';
+
+const createService = (overrides = {}) => {
+  const projectConfigRuntime = {
+    listScheduledTasks: vi.fn(async () => []),
+    deleteScheduledTask: vi.fn(async () => ({ deleted: true, tasks: [] })),
+    ...(overrides.projectConfigRuntime || {}),
+  };
+  const scheduledTasksRuntime = {
+    syncProject: vi.fn(async () => []),
+    ...(overrides.scheduledTasksRuntime || {}),
+  };
+  const service = createScheduledTaskService({
+    readSettingsFromDiskMigrated: async () => ({
+      projects: [{ id: 'project-test', path: '/repo' }],
+    }),
+    sanitizeProjects: (projects) => projects,
+    projectConfigRuntime,
+    scheduledTasksRuntime,
+  });
+  return { service, projectConfigRuntime, scheduledTasksRuntime };
+};
+
+const loopTask = {
+  id: 'loop:project:daily-digest',
+  name: 'daily-digest',
+  enabled: true,
+  loopFile: '/repo/.agents/loops/daily-digest.md',
+  schedule: { kind: 'cron', cron: '0 9 * * *', timezone: 'UTC' },
+  execution: { prompt: 'digest', providerID: 'openai', modelID: 'gpt-4.1' },
+};
+
+describe('scheduled-task service remove', () => {
+  it('rejects deleting a loop-sourced task without touching storage', async () => {
+    const { service, projectConfigRuntime, scheduledTasksRuntime } = createService({
+      projectConfigRuntime: {
+        listScheduledTasks: vi.fn(async () => [loopTask]),
+      },
+    });
+
+    await expect(service.remove('project-test', loopTask.id)).rejects.toMatchObject({
+      statusCode: 400,
+      message: expect.stringContaining('delete the file to remove the task'),
+    });
+    expect(projectConfigRuntime.deleteScheduledTask).not.toHaveBeenCalled();
+    expect(scheduledTasksRuntime.syncProject).not.toHaveBeenCalled();
+  });
+
+  it('deletes JSON-configured tasks normally', async () => {
+    const jsonTask = { ...loopTask, id: 'json-task', loopFile: undefined };
+    const { service, projectConfigRuntime, scheduledTasksRuntime } = createService({
+      projectConfigRuntime: {
+        listScheduledTasks: vi.fn(async () => [jsonTask]),
+        deleteScheduledTask: vi.fn(async () => ({ deleted: true, tasks: [] })),
+      },
+    });
+
+    const tasks = await service.remove('project-test', jsonTask.id);
+
+    expect(projectConfigRuntime.deleteScheduledTask).toHaveBeenCalledWith('project-test', jsonTask.id);
+    expect(scheduledTasksRuntime.syncProject).toHaveBeenCalled();
+    expect(Array.isArray(tasks)).toBe(true);
+  });
+});