Add production protocol connector ADR

[smithaldon1]

Closes #210

Summary

• Add ADR 0002 defining the read-only production connector boundary for OPC-UA, MQTT, and BACnet.
• Document connection profile ownership, storage expectations, lifecycle states, secret/certificate references, test-connection behavior, and audit/logging expectations.
• Add focused documentation tests plus a learning-log entry for the connector decision.

Test Evidence

• .venv/bin/python -m pytest services/simulator/tests/test_production_connector_adr_docs.py - passed, 4 tests.
• make lint - passed.
• make typecheck - passed.
• make test - passed, 213 tests.
• make docs - passed placeholder docs target: "Docs are Markdown-only in the MVP skeleton; no docs checker is configured yet."
• git diff --check - passed.

Docs Impact

• Added docs/decisions/0002-production-protocol-connector-architecture.md.
• Linked ADR 0002 from docs/decisions/README.md.
• Updated docs/LEARNING_LOG.md.

Follow-up Work

• Implement 2. Feature: Define protocol connection profile schema #207 next: shared protocol connection profile schema.
• Keep adapter, API, and UI work out of this ADR PR.

[smithaldon1]

Reviewed and approved for merge