GCP-05: Firestore Sibling Context — Kill Cold Start Tax

Replace MEMORY.md files (200-line truncated markdown) with Firestore for structured sibling context and shared state. Current pain: Cold start burns ~30% of context window re-reading MEMORY.md + 18 messages. MEMORY.md already truncated at 200 lines (losing data). No real-time sync. Architecture: 1. Firestore database: c8-sibling-mesh (Native mode) 2. Collections: siblings/{id}/context, siblings/{id}/memory, shared/rosetta-core, shared/foundry, shared/seko 3. Cold start: 3 Firestore reads (~2KB) vs current 1 file + 18 REST calls 4. Write-through to both Firestore AND existing systems during migration 5. Real-time listeners: Cloud Lu subscribes to siblings/*/context Deliverables: - sibling_context.py (read/write/subscribe) - Migration script: MEMORY.md -> Firestore documents - get_cold_start_context(instance_id) helper - Firestore security rules Target: c8_platform/sibling_context.py + c8-infra/firestore/