feat: add doctor baseline command

[CoderDeltaLAN]

Summary

• Adds the read-only doctor baseline command.
• Reuses existing discovery and governance diagnostics for repository-level summaries.
• Documents the doctor output and exit-code contract.
• Adds CLI and golden output coverage.

Verification

• PYTHONDONTWRITEBYTECODE=1 PYTHONPATH=src python -m unittest tests.test_cli tests.test_golden_outputs
• ./scripts/check.sh
• PYTHONPATH=src python -m agent_rules_kit.cli doctor tests/fixtures/repositories/risky-instructions

Scope

• No network or LLM runtime behavior.
• No repository command execution.
• No CI, GitHub branch protection, dependency, or security certification audit.