Add UV-5R Mini support and CHIRP CSV export

[emuehlstein]

Summary

This PR adds first-class support for baofeng_uv5r_mini and introduces a CHIRP CSV export path for analog radios.

Changes

• Add UV-5R Mini capabilities document:
  • radios/baofeng_uv5r_mini/capabilities.json
• Add UV-5R Mini reference fixture profile:
  • profiles/baofeng_uv5r_mini/reference.yml
• Add CHIRP exporter module:
  • src/codeplugger/exporters/chirp_csv.py
  • src/codeplugger/exporters/__init__.py
• Add CLI output format:
  • codeplugger-profile --output-format chirp-csv
• Add tests:
  • tests/test_chirp_csv.py
  • profile validation and FM-only mode checks in tests/test_profile.py
• Add docs:
  • CHIRP workflow section in README.md
• Ignore .DS_Store in .gitignore

Behavior

• CHIRP CSV export is deterministic and preserves resolved channel order.
• FM-only path is enforced for CHIRP export (non-FM channels fail with explicit errors).
• TX-disabled channels map to CHIRP Duplex=off.

Validation

• pytest -q
• Result: 18 passed

Follow-up

• Issue Support a radio_instance registry for physical-radio facts (profile-0.2) #8 work (instance registry) is being developed in a separate branch to keep scope isolated.