Add auth resilience: user cache, retry, and API token fallback ...

- 30-second in-memory _user_cache to avoid hammering gnexus-auth
- _fetch_user_with_retry: one retry after 1.5s sleep on transient failure
- API token fallback when OAuth cookie is present but refresh fails
- Clear cache/locks in test fixture to prevent cross-test pollution
- Fix registry timeout test after lowering default to 90s

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Fix auth race condition causing frequent logouts ...

Add per-session-id asyncio.Lock around token refresh to prevent
parallel requests from simultaneously refreshing the same token.
Re-read the session inside the lock so a second request can use
the token already refreshed by the first one.

Stop deleting the auth session on refresh failure — transient
errors (network, race condition, expired refresh token) were
wiping the session and forcing a full re-login.

+ tests for both behaviours.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Extract ContextCompressor, fix STL viewer, expand test suite, add architecture audit docs ...

- Extract ContextCompressor from agent.py (Step 1 of god-object refactor)
- Add retry + hard-truncate fallback logic to ContextCompressor
- Add unit tests: agent loop (14), compressor (18), KV store (8),
  auth encrypt (3), auth deps (13), todo/scratchpad/image_view/memory
- Fix WebGL STL viewer: allow-same-origin sandbox + graceful fallback
- Add CompressionStarted event and client-side compression notice
- Add docs/architecture_weak_spots.md and plan_01_god_object_agent.md
- Update test_events.py and test_agent_context_size.py for new logic

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>