feat(tasks): support markdown scheduled-task loops in .agents/loops

Adds markdown-based scheduled-task definitions ("loops") discovered from
.agents/loops/*.md (project scope, ancestor directories up to the
worktree root) and ~/.agents/loops/*.md (user scope), mirroring the
skills discovery pattern.

File format: YAML frontmatter (name, schedule cron, enabled, model as
provider/model, optional agent/timezone) plus the markdown body as the
execution prompt. Discovery and parsing live in
scheduled-tasks/loops.js; project-config gains reconcileLoopTasks which
runs inside the project write lock on every syncProject:
- identity by task name; a loop takes over a matching task, preserving
  its id and runtime state (markdown wins on conflict with JSON)
- tasks whose loopFile is gone are unscheduled; JSON tasks are never
  removed
- new loops are created under deterministic loop:<scope>:<name> ids
- project scope shadows user scope on name collisions
- malformed files are skipped with a warning and never block valid ones

Runtime state stays in the project config/state store; it is never
written to the markdown files. Module documentation updated with the
file format and reconciliation rules.

Fixes #2583

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

@@ -3,6 +3,7 @@ import { DateTime } from 'luxon';
 import parser from 'cron-parser';
 import { expandSnippets } from '../opencode/snippets.js';
 import { buildGoalIntroText, createSessionGoal } from '../session-goal/create.js';
+import { discoverLoops } from './loops.js';
 
 const DEFAULT_GLOBAL_CONCURRENCY = 4;
 const DEFAULT_PROJECT_CONCURRENCY = 2;
@@ -382,8 +383,19 @@ export const createScheduledTasksRuntime = (deps) => {
 
   const syncProject = async (projectID) => {
     await ensureProjectPath(projectID);
+    const projectPath = projectPathByID.get(projectID) || null;
+
+    let tasks;
+    if (projectPath) {
+      // Reconcile `.agents/loops` definitions with the persisted task list:
+      // loop files are authoritative while present, removed files unschedule
+      // their task, and runtime state is preserved (see loops.js).
+      const loops = await discoverLoops(projectPath);
+      tasks = await projectConfigRuntime.reconcileLoopTasks(projectID, loops);
+    } else {
+      tasks = await projectConfigRuntime.listScheduledTasks(projectID);
+    }
 
-    const tasks = await projectConfigRuntime.listScheduledTasks(projectID);
     setProjectTasks(projectID, tasks);
 
     for (const task of tasks) {