memory: degrade safety — warn/refuse in-memory pgvector writes + surface degraded status + document the gate

[thinmintdev]

What to build

Docs present Cognee/memory as default-on, but code gates it behind HAL0_MEMORY_ENABLED (default off), and the degrade fallback pgvector_provider is an in-memory stub that drops all writes on restart. A user who trusts the docs, flips the flag, and lands on degrade loses data silently. Make degraded memory loud.

Pointers: memory/pgvector_provider.py, memory/__init__.py:89, api/__init__.py memory gate. Audit 0.5 / Q4.3.

Acceptance criteria

• Degraded (in-memory) memory writes emit a loud warning, or are refused outright
• A distinct "memory degraded (in-memory only)" status is surfaced (API + dashboard)
• The HAL0_MEMORY_ENABLED gate + degrade behavior are documented

Blocked by

None - can start immediately