chore: add quality tooling and CI

[mgratzer]

Summary

Add automated quality checks for the forge repository — markdown linting, frontmatter validation, and a CI workflow to enforce them on PRs.

Problem / Motivation

Forge is a markdown-only repo with no automated quality gates. Style drift, broken heading hierarchy, and inconsistent formatting across SKILL.md files can only be caught by manual review. Adding linting and CI ensures consistent quality as the skill collection grows.

Sub-Issues

• chore(lint): add markdownlint-cli2 configuration #2 — Add markdownlint-cli2 configuration
• chore(ci): add GitHub Actions workflow for PR checks #3 — Add GitHub Actions workflow for PR checks
• chore(ci): add YAML frontmatter validation for SKILL.md files #4 — Add YAML frontmatter validation for SKILL.md files

Acceptance Criteria

• Markdown linter configured and passing on all files
• YAML frontmatter validation for all SKILL.md files
• CI workflow runs all checks on every PR
• CLAUDE.md and docs updated with lint commands