Add CONTRIBUTING.md, PR validation CI, and eval framework

[Copilot]

No contributor guide, no structural validation CI, and no eval framework for testing agent/skill behavior. Community PRs have no automated verification pipeline.

CONTRIBUTING.md

• Skill contribution guide: directory structure, SKILL.md frontmatter schema, naming conventions
• Agent changes are maintainer-curated (discussion-first model)
• PR process and what CI validates
• Microsoft Open Source Code of Conduct linked

PR Validation Workflow (.github/workflows/pr-validation.yml)

Active workflow running on every PR to main:

• Structural validation (scripts/validate-structure.js): frontmatter required fields, name-directory consistency, kebab-case enforcement, SKILL.md presence, required sections (## When to Use, ## Procedure), agent cross-reference integrity, relative link resolution
• Markdownlint with .markdownlint.json config

All checks pass on current repo state (section warnings are non-blocking for equivalent headings like ## Execution Playbook).

Eval Framework (evals/)

Decision: custom Node.js harness over openai/evals (wrong ecosystem, Python-only, model-coupled).

• evals/README.md — investigation summary and decision rationale
• evals/scenarios/deploy-function-app.json — core deployment path scenario
• evals/run-eval.js — validates scenarios reference real agents/skills from repo definitions

{
  "id": "deploy-function-app",
  "expected_agent": "Git-Ape",
  "expected_skill_sequence": ["prereq-check", "azure-naming-research", "..."],
  "assertions": [
    { "type": "skill_order", "before": "azure-deployment-preflight", "after": "azure-integration-tester" }
  ]
}

[sendtoshailesh]

Closing in favor of a re-scoped PR. Issue #17 has been narrowed to CONTRIBUTING.md + structural PR validation CI, and the exploratory eval framework portion is now tracked in #61. The replacement PR (scoped to the reduced #17) is being opened next and carries forward the CONTRIBUTING.md, scripts/validate-structure.js, .github/workflows/pr-validation.yml, and .markdownlint.json from this branch. The evals/ artifacts here are not lost — #61 references them as prior art.