feat(P4-003): @mosaic/log — log ingest, parsing, tiered storage

Implement AgentLogsRepo with structured log ingest (single + batch),
flexible query builder (filter by session, level, category, tier,
date range), and tiered storage management (hot→warm→cold→purge).
Add getLogsForSummarization() for the summarization pipeline.
Wire LogModule into gateway with REST endpoints at /api/logs.

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

apps/gateway/package.json

@@ -18,6 +18,7 @@
     "@mosaic/brain": "workspace:^",
     "@mosaic/coord": "workspace:^",
     "@mosaic/db": "workspace:^",
+    "@mosaic/log": "workspace:^",
     "@mosaic/memory": "workspace:^",
     "@mosaic/types": "workspace:^",
     "@nestjs/common": "^11.0.0",