chore(ci): add YAML frontmatter validation for SKILL.md files

[mgratzer]

Parent: #1

Summary

Add a validation check that ensures every skills/*/SKILL.md file has valid YAML frontmatter with required fields and that the name field matches its directory name.

Problem / Motivation

If a skill's name field in the YAML frontmatter doesn't match its directory name, or if required fields are missing, the skill won't be discoverable or will behave unexpectedly when invoked. This is invisible until someone tries to use the skill.

Proposed Solution

A bash script (or CI step) that validates every SKILL.md frontmatter.

Implementation Details

1. Required fields check: Every SKILL.md must have name and description in frontmatter
2. Name-directory match: name value must equal the parent directory name (e.g., skills/forge-setup-project/SKILL.md → name: forge-setup-project)
3. Optional fields validation: If allowed-tools is present, verify it's a comma-separated list of known tool names
4. Integration: Add as a step in the CI workflow from chore(ci): add GitHub Actions workflow for PR checks #3, or as a standalone script in scripts/

Alternatives Considered

• Schema validation with a YAML validator — More robust but adds a dependency. A bash script with grep/sed is sufficient for the small number of fields.
• Fold into CI issue chore(ci): add GitHub Actions workflow for PR checks #3 — Kept separate for cleaner scope since this is a distinct validation concern.

Acceptance Criteria

• Validation script or CI step created
• Checks name and description are present in every SKILL.md
• Checks name matches directory name
• Fails with a clear error message on violations
• Runs as part of CI workflow (chore(ci): add GitHub Actions workflow for PR checks #3)