test(docs): add regression tests for skill validators

[kubestellar-hive]

Test Improvement

Adds regression coverage for docs validator scripts for broken relative links, missing skill front matter, skill catalog generation, and stale index detection.

Also wires new test module into just test python pytest invocation.

Related Issue

Fixes #910

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Filed by quality agent (hold-gated mode). Human review required.

[clubanderson]

Marking for hold-gated flow: please apply labels quality, testing, and hold. This PR is not for merge automation.

[castrojo]

Closing as redundant with coverage already on main.

#911 (merged) already added tests for three of the four areas this PR covers, in tests/test_skill_docs.py:

• check_doc_links → test_check_doc_links_passes_with_complete_catalog, test_check_doc_links_reports_missing_link
• check_skill_frontmatter → test_check_skill_frontmatter_passes_with_valid_files, test_check_skill_frontmatter_reports_missing_front_matter
• generate_skill_index → test_generate_skill_index_round_trip, test_generate_skill_index_rejects_missing_entry_point, plus stale-date coverage added in fix(ci): stop validate failing on stale generated_at date #937

Landing this would create a second parallel test file (tests/test_docs_skill_validators.py) duplicating the same validators, which splits maintenance across two files.

The one genuinely uncovered validator was check-skill-index — that gap is being closed by #924 instead. It's worth noting that gap was real: check-skill-index failed on main tonight (a missing queue-feed.md link in docs/SKILL.md), which failed the required validate job on every open PR until it was fixed.

Sibling PR #924 covers issue #910. No coverage is lost by closing this.