CONTRIBUTING.md and contribution verification CI

[arnaudlh]

Description

The repo has no CONTRIBUTING.md, no tests, and no contributor guide. Community members are already filing issues (#10, #11). Skills can be contributed via PR — but there's no verification pipeline to validate contributions.

Contribution model: Skills are community-contributable via PR. Agents are maintainer-curated.

Scope change: The exploratory Agent/Skill eval framework originally bundled here has been split into #61 so this issue can land focused on the contributor guide and structural PR validation. PR #40 (which mixed all three sections) is being closed and replaced with a new PR scoped to the items below.

Scope

1. CONTRIBUTING.md

Contributor guide covering:

• How to add a new skill (directory structure, SKILL.md schema, naming conventions).
• How to propose agent changes (maintainer-only, discussion first).
• PR process and what the CI checks validate.
• Code of Conduct (Microsoft Open Source standard).

2. Structural verification CI workflow

.github/workflows/pr-validation.yml — active workflow (not .example.yml). Automated checks on every PR:

• YAML frontmatter validation — required fields present (name, description for skills; description for agents), correct types.
• Name-directory consistency — skill name: in frontmatter matches its parent directory name.
• Kebab-case directory naming — all skill/agent directories follow kebab-case convention.
• SKILL.md presence — every skill directory contains a SKILL.md file.
• Required sections — skills have ## When to Use and ## Procedure; agents have ## Warning disclaimer.
• Cross-reference integrity — slash-command references (/skill-name) map to existing skill directories; agent agents: frontmatter entries match real agent name: fields.
• Relative link validation — Markdown links to local files resolve to real paths.
• Markdownlint — standard Markdown linting across all .md files.

Acceptance Criteria

• CONTRIBUTING.md exists with clear skill contribution guidelines and SKILL.md schema.
• Code of Conduct linked (Microsoft OSS standard).
• PR validation workflow runs on every PR (active .yml, not .example.yml).
• Frontmatter, naming, cross-reference, and section checks all pass on the current repo.

Split out

• Agent/Skill eval framework (exploratory) → Agent/Skill eval framework #61

[sendtoshailesh]

Scope reduced: section 3. Agent/Skill eval framework (exploratory) has been split out into #61. This issue now focuses on CONTRIBUTING.md + the structural PR validation workflow only. PR #40 (which mixed all three sections) is being closed; a new PR scoped to this reduced issue will replace it.