test(memory): stabilize memory retrieval suite

[HugoSkylight]

Summary

• Captured a full pytest baseline and fixed low-risk stability failures.
• Made live HTTP tests opt-in via OPENCORTEX_RUN_LIVE_TESTS=1 so stale local servers do not break the default suite.
• Removed stale MCP live tests from tests/test_live_servers.py; src/opencortex/mcp_server.py no longer exists.
• Updated Insights collector mock filtering, LocalEmbedder test construction, and e2e loop cleanup for fire-and-forget CortexFS writes.
• Removed low-risk warning sources from recall accuracy naming, datetime.utcnow(), and tests that accidentally created a real default LocalEmbedder.

Baseline

• Initial uv run --group dev pytest: 15 failed, 1263 passed, 34 skipped, 5 warnings in 222.04s.

Validation

• uv run --group dev pytest: 1270 passed, 34 skipped, 1 warning in 70.70s.
• uv run --group dev pytest tests/insights/test_collector.py tests/test_local_embedder.py tests/test_live_servers.py -q: 14 passed, 8 skipped.
• uv run --group dev pytest tests/test_memory_service.py tests/test_e2e_phase1.py -q: 54 passed.
• uv run --group dev pytest tests/test_perf_fixes.py tests/test_context_manager.py -q: 89 passed.
• uv run --group dev pytest tests/test_object_rerank.py tests/test_object_cone.py tests/test_retrieval_candidate_service.py -q: 5 passed.
• uv run --group dev ruff check .: passed.
• uv run --group dev ruff format --check .: passed.

Review

Residual actionable work: none.

Browser Gate

No frontend or route-mapped UI files changed; agent-browser gate is no-op.