docs(cli): list all supported LLM providers

[deepujain]

Summary

• Update skillspector scan --help to list every usable LLM provider supported by the selector.
• Document the missing provider credentials and local-auth behavior.
• Add a CLI regression test covering the provider names in rendered help.

Validation

• pytest tests/unit/test_cli.py -q — 102 passed.
• pytest -m 'not integration and not provider' tests/ -q — 2,826 passed, 13 skipped, 38 deselected, 4 xfailed.
• ruff check src tests — passed.
• ruff format --check src tests — 193 files already formatted.
• git diff --check — passed.
• Rendered skillspector scan --help and inspected the provider and credential sections.

Risk

• Low: this changes built-in help text and its regression coverage only; provider selection and authentication behavior are unchanged.
• Integration and live-provider lanes were not run because the change does not exercise provider requests.

Fixes #431