Docs: Document connector safety model

[smithaldon1]

Summary

Closes #219.

• Add docs/CONNECTOR_SAFETY_MODEL.md covering read-only connector defaults, OPC-UA/MQTT/BACnet profile assumptions, local Demo-Factory fixture endpoints, and forbidden writeback scope.
• Link the connector safety model from architecture, security, and development docs.
• Add a docs regression test so the required safety and fixture language stays present.
• Add the required learning-log summary for the connector-management direction.

Test Evidence

• .venv/bin/python -m pytest services/simulator/tests/test_connector_safety_model_docs.py -> 4 passed
• make test -> 277 passed
• make lint -> All checks passed!
• make typecheck -> compileall packages services passed
• make docs -> Docs are Markdown-only in the MVP skeleton; no docs checker is configured yet.
• git diff --check -> passed

Docs Impact

• Adds a new connector safety reference doc.
• Updates architecture/security/development docs to point contributors to the read-only connector boundary and local fixture assumptions.
• Updates docs/LEARNING_LOG.md.

Follow-up Work

• Keep future connector UI/API/adapter issues aligned with this read-only boundary.
• Any future write path should start with a separate ADR and governed-action design.

[smithaldon1]

Reviewed and approved for merge