feat: @mosaic/coord — migrate from v0, gateway integration (P2-005)

Migrate the orchestration coordination package from v0 with full
file-based mission/task lifecycle management. Adds gateway REST API
at /api/coord/* and agent tools for coord queries.

Package modules:
- types: Mission, task, session, milestone type definitions
- mission: Create, load, save missions with atomic file writes
- tasks-file: TASKS.md markdown table parsing/generation with file locking
- runner: Task execution subprocess spawning with session locks
- status: Mission and task status queries

Gateway integration:
- CoordModule with CoordService wrapping coord functions
- CoordController exposing status/tasks/task-detail endpoints
- Agent tools: coord_mission_status, coord_list_tasks, coord_task_detail

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/gateway/package.json

@@ -16,6 +16,7 @@
     "@mariozechner/pi-coding-agent": "~0.57.1",
     "@mosaic/auth": "workspace:^",
     "@mosaic/brain": "workspace:^",
+    "@mosaic/coord": "workspace:^",
     "@mosaic/db": "workspace:^",
     "@mosaic/types": "workspace:^",
     "@nestjs/common": "^11.0.0",