feat: add api-mgmt command

[TacoRocket]

Summary

• add the Phase 3 api-mgmt command with service-level gateway, hostname, identity, and exposure posture
• keep the first slice narrow and park deeper APIM subscription, named-value secret, Key Vault, and policy-body depth in future candidates
• add fixture, schema, golden, help, and terminal UX coverage for the new command

Validation

• python3 -m ruff check src tests scripts
• PYTHONPATH=src python3 scripts/generate_schemas.py
• PYTHONPATH=src python3 -m pytest tests/test_collectors.py tests/test_golden_outputs.py tests/test_contract_schemas.py tests/test_cli_smoke.py tests/test_help.py tests/test_terminal_ux.py tests/test_models.py
• PYTHONPATH=src python3 -m pytest