docs: refresh contributor documentation

[anderskev]

Summary

Refresh Beagle contributor and repository guidance after the Agent Skills migration.

Type of change

• New skill
• New command
• Skill improvement
• Command improvement
• Bug fix
• Documentation
• Other: ___

Changes

• Update CONTRIBUTING.md with current skill layout, validation commands, and release steps.
• Correct AGENTS.md and CLAUDE.md skill counts and marketplace manifest details.
• Make PRIVACY.md agent-agnostic instead of Claude-Code-only.

Checklist

• Follows contribution guidelines
• Skill/command has valid YAML frontmatter
• Description includes trigger keywords (for skills)
• Tested manually with Claude Code
• Updated README.md if adding new skill/command
• Updated CLAUDE.md command count if applicable

Testing

• ./scripts/check-portability.sh
• python3 frontmatter validation over plugins//skills//SKILL.md
• active skill inventory check for 132 active skills
• changed-doc relative link check
• git diff --check

Related issues

Closes #

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 600a44ae-d9bb-4b6d-89bf-7d6d77c9fadf

📥 Commits

Reviewing files that changed from the base of the PR and between d24de2d and b5cdbc2.

📒 Files selected for processing (4)

• AGENTS.md
• CLAUDE.md
• CONTRIBUTING.md
• PRIVACY.md

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Walkthrough

The PR updates documentation to describe a centralized .claude-plugin/marketplace.json manifest, active plugins as directories under plugins/, canonical SKILL.md entry points, and revised skill authoring, validation, commit, pull request, and release instructions. It also rewrites PRIVACY.md to use generic Beagle/agent-skills marketplace language and updated local tool-usage wording.

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_{Comment @coderabbitai help to get the list of available commands.}