docs(skill): point install guidance at docs README

[bearomorphism]

Description

Follow-up to #1949 addressing review feedback from @philipp-horstenkamp:

Shouldn't this be pipx or uv?

The merged .agents/skills/commitizen/SKILL.md advertised pip install commitizen as the install command, but docs/README.md recommends pipx install commitizen or uv tool install commitizen for global installs of the cz CLI; pip install -U commitizen is documented only as a project-specific install.

Changes

• Frontmatter install: updated from pip install commitizen to pipx install commitizen to match the docs' primary recommendation.
• Replaced the inline install command list in Important domain details (which had the same pip/uv add discrepancy) with a short pointer that defers to docs/README.md's Installation section.
• Added an Installation: entry under Suggested references, matching the file's existing pattern of referencing docs/...md.

Why a pointer instead of duplicating the matrix

Keeping the full install command list in two places was the root cause of the original review comment: the SKILL.md had drifted from the docs. Pointing at docs/README.md makes the docs the single source of truth. The frontmatter still carries one canonical command because the Agent Skills spec expects a literal command there.

Why not extract a separate docs/installation.md

That was considered, but it expands scope (mkdocs nav + llmstxt config + risk of breaking external links to the README's #installation anchor) beyond the original review concern. It can be a separate proposal if desired.

Checklist

• I have read the contributing guidelines

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

• Yes (GitHub Copilot CLI)

Generated-by: GitHub Copilot CLI following the guidelines

Code Changes

• Add test cases to all the changes you introduce (N/A - docs only)
• Run uv run poe all locally to ensure this change passes linter check and tests (no Python files touched; pre-commit hooks pass on push)
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios (verified install commands match docs/README.md lines 62-122)
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
• Update the documentation for the changes

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly (only .agents/skills/commitizen/SKILL.md changed, not part of the mkdocs site)
• Check and fix any broken links (internal or external) - verified docs/README.md reference is valid

Steps to Test This Pull Request

1. Open .agents/skills/commitizen/SKILL.md.
2. Confirm the frontmatter install: is pipx install commitizen.
3. Confirm the Important domain details item points to docs/README.md rather than listing install commands inline.
4. Confirm Suggested references contains an Installation entry pointing to docs/README.md.

Additional Context

Addresses unresolved review feedback from #1949 (already merged).