feat: MACP Phase 2A — Event Bridge + Notification System (#11)

docs/tasks/MACP-PHASE2A-brief.md

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+# MACP Phase 2A — Event Bridge + Notification System
+
+**Branch:** `feat/macp-phase2a`
+**Repo worktree:** `~/src/mosaic-bootstrap-worktrees/macp-phase2a`
+
+---
+
+## Objective
+
+Build the event bridge that makes MACP events consumable by external systems (OpenClaw, Discord, webhooks). This is the observability layer — the controller already writes events to `events.ndjson`, but nothing reads them yet.
+
+---
+
+## Task 1: Event File Watcher (`tools/orchestrator-matrix/events/event_watcher.py`)
+
+New Python module that tails `events.ndjson` and fires callbacks on new events.
+
+### Requirements:
+- Watch `.mosaic/orchestrator/events.ndjson` for new lines (use file polling, not inotify — keeps it portable)
+- Parse each new line as JSON
+- Call registered callback functions with the parsed event
+- Support filtering by event type (e.g., only `task.completed` and `task.failed`)
+- Maintain a cursor (last read position) so restarts don't replay old events
+- Cursor stored in `.mosaic/orchestrator/event_cursor.json`
+
+### Key Functions:
+```python
+class EventWatcher:
+    def __init__(self, events_path: Path, cursor_path: Path, poll_interval: float = 2.0):
+        ...
+    
+    def on(self, event_types: list[str], callback: Callable[[dict], None]) -> None:
+        """Register a callback for specific event types."""
+    
+    def poll_once(self) -> list[dict]:
+        """Read new events since last cursor position. Returns list of new events."""
+    
+    def run(self, max_iterations: int = 0) -> None:
+        """Polling loop. max_iterations=0 means infinite."""
+```
+
+### Constraints:
+- Python 3.10+ stdlib only (no pip dependencies)
+- Must handle truncated/corrupt lines gracefully (skip, log warning)
+- File might not exist yet — handle gracefully
+- Thread-safe cursor updates (atomic write via temp file rename)
+
+---
+
+## Task 2: Webhook Adapter (`tools/orchestrator-matrix/events/webhook_adapter.py`)
+
+POST events to a configurable URL. This is how the OC plugin will consume MACP events.
+
+### Requirements:
+- Accept an event dict, POST it as JSON to a configured URL
+- Support optional `Authorization` header (bearer token)
+- Configurable from `.mosaic/orchestrator/config.json` under `macp.webhook`:
+  ```json
+  {
+    "macp": {
+      "webhook": {
+        "enabled": false,
+        "url": "http://localhost:8080/macp/events",
+        "auth_token": "",
+        "timeout_seconds": 10,
+        "retry_count": 2,
+        "event_filter": ["task.completed", "task.failed", "task.escalated"]
+      }
+    }
+  }
+  ```
+- Retry with exponential backoff on failure (configurable count)
+- Log failures but don't crash the watcher
+- Return success/failure status
+
+### Key Functions:
+```python
+def send_webhook(event: dict, config: dict) -> bool:
+    """POST event to webhook URL. Returns True on success."""
+
+def create_webhook_callback(config: dict) -> Callable[[dict], None]:
+    """Factory that creates a watcher callback from config."""
+```
+
+### Constraints:
+- Use `urllib.request` only (no `requests` library)
+- Must not block the event watcher for more than `timeout_seconds` per event
+- Log to stderr on failure
+
+---
+
+## Task 3: Discord Notification Formatter (`tools/orchestrator-matrix/events/discord_formatter.py`)
+
+Format MACP events into human-readable Discord messages.
+
+### Requirements:
+- Format functions for each event type:
+  - `task.completed` → "✅ **Task TASK-001 completed** — Implement user auth (attempt 1/1, 45s)"
+  - `task.failed` → "❌ **Task TASK-001 failed** — Build error: exit code 1 (attempt 2/3)"
+  - `task.escalated` → "🚨 **Task TASK-001 escalated** — Gate failures after 3 attempts. Human review needed."
+  - `task.gated` → "🔍 **Task TASK-001 gated** — Quality gates running..."
+  - `task.started` → "⚙️ **Task TASK-001 started** — Worker: codex, dispatch: yolo"
+- Include task metadata: runtime, dispatch type, attempt count, duration (if available)
+- Keep messages concise — Discord has character limits
+- Return plain strings (the caller decides where to send them)
+
+### Key Functions:
+```python
+def format_event(event: dict) -> str | None:
+    """Format an MACP event for Discord. Returns None for unformattable events."""
+
+def format_summary(events: list[dict]) -> str:
+    """Format a batch summary (e.g., daily digest)."""
+```
+
+---
+
+## Wiring: CLI Integration
+
+Add to `bin/mosaic-macp`:
+```bash
+mosaic macp watch [--webhook] [--once]
+```
+- `watch`: Start the event watcher with configured callbacks
+- `--webhook`: Enable webhook delivery (reads config from `.mosaic/orchestrator/config.json`)
+- `--once`: Poll once and exit (useful for cron)
+
+---
+
+## Verification
+
+1. Create a test `events.ndjson` with sample events, run `event_watcher.poll_once()`, verify all events returned
+2. Run watcher with webhook pointing to a local echo server, verify POST payload
+3. Format each event type through `discord_formatter`, verify output strings
+4. `mosaic macp watch --once` processes events without error
+5. Cursor persistence: run twice, second run returns no events
+
+## File Map
+```
+tools/orchestrator-matrix/
+├── events/
+│   ├── __init__.py               ← NEW
+│   ├── event_watcher.py          ← NEW
+│   ├── webhook_adapter.py        ← NEW
+│   └── discord_formatter.py      ← NEW
+```
+
+## Ground Rules
+- Python 3.10+ stdlib only
+- No async/threads — synchronous polling
+- Commit: `feat: add MACP event bridge — watcher, webhook, Discord formatter`
+- Push to `feat/macp-phase2a`