step-7: add memory middleware (memory_middleware.py, device_ws.py)

MemoryMiddleware class:
- enrich_context(): loads core prefs, associative (top-k), episodic (last-N),
  and proactive hints (above 0.6 confidence) — all decrypted in-memory only
- store_episode(): encrypts and persists interaction summary to memory_episodic
- update_core(): upserts encrypted key/value to memory_core

device_ws.py home_request + popup_request handlers:
- enrich_context() called before orchestrate_v3_stream (memory injected into context)
- store_episode() called after stream completes (non-blocking)

10 unit + integration tests pass; pre-existing test_agents.py failures unrelated.

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

V3_MIGRATION_PLAN.md

@@ -328,7 +328,7 @@ pytest tests/test_memory_middleware.py
 ```
 
 **Status**:
-- [ ] Step 7 complete
+- [x] Step 7 complete
 
 **Commit**: After tests pass, commit with:
 ```