docs: codify description guidelines and clean up contributing doc redundancy

[patternfly-jira-sync]

• Codify the skill description formula ([Action verb] [what it does]. [Use when + trigger contexts.]) in CONTRIBUTING-SKILLS.md — we've been applying it across all 28 skills but it's undocumented

• Remove duplicated content between CONTRIBUTING.md and CONTRIBUTING-SKILLS.md (plugin naming standard and skills vs agents sections appear in both)

• Pre-requisite cleanup before the plugin taxonomy restructure

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Jira Issue: PF-4156

[coderabbitai]

🔗 Related PRs

#6 - feat(skills): add PatternFly 6 development skills for Claude Code [merged]
#26 - feat(skill): summarize jira issues & PR reviews [merged]
#45 - feat(pf-design-tokens): add analyze-modifiers skill [merged]
#48 - Add Jira quarterly-initiative-report skill to pf-workflow plugin [merged]
#59 - chore(plugins): rename design-tokens to design-foundations, relocate skills [merged]

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[patternfly-jira-sync]

Closed via Jira sync - Jira issue PF-4156 was closed.