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/DOCUMENTATION.md

@@ -5,7 +5,8 @@ Server-owned scheduled task runtime and routes for OpenChamber-only automation.
 ## Scope
 
 - Per-project scheduled task persistence is owned by `packages/web/server/lib/projects/project-config.js`.
-- Runtime orchestration and execution is owned by this module.
+- Markdown loop discovery/parsing is owned by `packages/web/server/lib/scheduled-tasks/loops.js`.
+- Runtime orchestration and execution is owned by `packages/web/server/lib/scheduled-tasks/runtime.js`.
 - This module is OpenChamber feature logic; it is intentionally separate from OpenCode proxy/runtime internals.
 
 ## Files
@@ -17,11 +18,74 @@ Server-owned scheduled task runtime and routes for OpenChamber-only automation.
   - Session create + prompt_async execution
   - Emits OpenChamber task-run events
 
+- `packages/web/server/lib/scheduled-tasks/loops.js`
+  - Discovery of `.agents/loops/*.md` (project scope, ancestors up to the worktree root) and `~/.agents/loops/*.md` (user scope)
+  - Frontmatter parsing into scheduled-task definitions
+  - `syncProject` reconciles discovered loops with the persisted task list on every project sync (startup, task save/delete)
+
 - `packages/web/server/lib/scheduled-tasks/routes.js`
   - Scheduled task CRUD endpoints
   - Manual run endpoint
   - OpenChamber events SSE stream endpoint
 
+## Loop file format
+
+Portable, git-commit-able scheduled-task definitions:
+
+```markdown
+---
+name: daily-digest
+schedule: "0 9 * * *"
+enabled: true
+model: anthropic/claude-sonnet-4-5
+agent: plan
+timezone: Europe/Kyiv
+---
+Summarize repository changes since yesterday.
+```
+
+Field mapping (model: `packages/ui/src/lib/scheduledTasksApi.ts`):
+
+| Frontmatter | Task field |
+|---|---|
+| `name` | `name` (required) |
+| `schedule` | `schedule.kind: "cron"` + `schedule.cron` (required, cron-only in the portable format) |
+| `enabled` | `enabled` (default `true`) |
+| `model` | split on the first `/` into `execution.providerID` / `execution.modelID` (required) |
+| `agent` | `execution.agent` (optional) |
+| `timezone` | `schedule.timezone` (optional, IANA; defaults to the server zone) |
+| body | `execution.prompt` (required) |
+
+`thinking_level` and `goalEnabled`/`goalTokenBudget` are not part of the portable
+format (UI/JSON-only today); `daily`/`weekly`/`once` schedules remain UI/JSON-only.
+Runtime state (`lastRunAt`, `nextRunAt`, `lastStatus`, `lastError`, `lastSessionId`,
+`lastDurationMs`) is never written to the markdown file — it continues to live in
+the project config state store.
+
+## Loop reconciliation rules
+
+`projectConfigRuntime.reconcileLoopTasks(projectID, loops)` runs inside the
+project write lock on every `syncProject` when the project path is known:
+
+- **Identity is the task name.** A loop whose name matches an existing task
+  takes that task over: its schedule/execution/enabled are overwritten from the
+  file while the task's `id` and runtime `state` are preserved (markdown wins
+  on conflict with JSON-configured tasks).
+- **Deletion.** A task carrying the `loopFile` marker whose loop file is no
+  longer discovered (removed or renamed) is unscheduled (removed from the
+  config). The marker is persisted in the config file, so removal is detected
+  across restarts. JSON-configured tasks without the marker are never removed.
+- **Creation.** Loops without a matching task are created under a deterministic
+  `loop:<scope>:<name>` id so runtime state survives restarts.
+- **Scope precedence.** Project-scope loops shadow user-scope loops with the
+  same name; among project files the nearest ancestor wins.
+- **Malformed files** (missing `name`/`schedule`/`model`/body, invalid cron,
+  unreadable) are skipped with a warning and never block valid loops in the
+  same or other scopes.
+- **UI edits** to a loop-sourced task are preserved in the config but the loop
+  file remains authoritative: the next reconciliation re-applies the file's
+  definition (including `enabled`). Use `enabled: false` in the file to disable.
+
 ## Public exports (runtime.js)
 
 - `createScheduledTasksRuntime(dependencies)`