docs(skill): remove broken doc references and unused compatibility field

[bearomorphism]

Description

Cleans up .agents/skills/commitizen/SKILL.md based on feedback in #1936 (comment) and the Agent Skills specification.

• Remove the Suggested references section. The section listed paths like docs/commands/commit.md that are relative to the repo root, not the skill root. Per the spec, file references in a SKILL.md should be relative to the skill root, and skill folders are typically distributed standalone, so those references could not resolve. The canonical docs URL remains discoverable via metadata.docs in the frontmatter.
• Drop the compatibility frontmatter field. The spec notes that "most skills do not need" this field, and the previous value (Git repository with Python and Commitizen available...) did not describe a constraint that would actually prevent the skill from running.

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: GitHub Copilot CLI (Claude Opus 4.7) following the guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

N/A: no code changes; only updates to .agents/skills/commitizen/SKILL.md. No new test cases needed. SKILL.md frontmatter still validates as YAML and the body still parses as Markdown.

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external)

SKILL.md is not part of the MkDocs site; it is consumed by Agent Skills clients. The change removes broken links rather than introducing new ones, and the metadata.docs URL was verified to point at the live docs site.

Expected Behavior

• Agent Skills clients that load .agents/skills/commitizen/SKILL.md no longer see references to docs/... paths that do not exist when the skill folder is installed standalone.
• The frontmatter remains spec-valid (no required fields removed), so existing clients continue to discover and activate the skill.
• Agents looking for canonical documentation can still find it via metadata.docs (https://commitizen-tools.github.io/commitizen/).

Steps to Test This Pull Request

1. Check out this branch and open .agents/skills/commitizen/SKILL.md.
2. Confirm the YAML frontmatter parses (e.g. python -c "import yaml,pathlib,re; t=pathlib.Path('.agents/skills/commitizen/SKILL.md').read_text(); fm=re.match(r'---\n(.*?)\n---', t, re.S).group(1); yaml.safe_load(fm)").
3. Confirm there is no compatibility: key and no ## Suggested references section.
4. Validate against the spec rules in https://agentskills.io/specification.md (required name and description are present and within length limits).

Additional Context

• Follow-up to comment in Provide a skill for agentic coding #1936: Provide a skill for agentic coding #1936 (comment)
• Agent Skills spec (file references must be relative to the skill root, compatibility is optional and most skills do not need it): https://agentskills.io/specification.md