docs: add comprehensive documentation and tests for altimate-code check command

- Add `docs/docs/usage/check.md` with full documentation covering all 7 check
  types, CLI options, JSON output schema, policy file format, schema file format,
  severity levels, CI/CD integration (GitHub Actions, pre-commit, GitLab CI),
  and real-world usage examples
- Update `docs/docs/usage/cli.md` to reference the `check` command in the
  subcommands table
- Add `check` page to `docs/mkdocs.yml` navigation under Interfaces
- Extract helper functions (`normalizeSeverity`, `filterBySeverity`,
  `toCategoryResult`, `formatText`, `buildCheckOutput`) into
  `check-helpers.ts` for testability
- Refactor `check.ts` to import from `check-helpers.ts` (no behavior change)
- Add 53 unit tests covering: severity normalization, severity filtering,
  category result building, text formatting, output assembly, constants
  validation, and edge cases

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: anandgupta42