fix: repair workflow YAML parsing and lint CI

[itstimwhite]

Summary

• rewrite the two diff-check workflow steps as block scalars so GitHub can parse the workflow YAML
• preserve the existing skill docs freshness behavior for Claude and Codex generation
• add a lightweight actionlint workflow to catch future workflow syntax regressions

Verification

• ruby YAML parse for .github/workflows/skill-docs.yml
• ruby YAML parse for .github/workflows/actionlint.yml

Notes

• left unrelated local changes untouched (bin/gstack-config, pnpm-lock.yaml)

[itstimwhite]

Verified on the fork per CONTRIBUTING.md fork-first flow.

Validation on itstimwhite/gstack:itstimwhite/first-oss-bugfix:

• Skill Docs Freshness push run 23333261727: success
• Workflow Lint push run 23333261707: success

Root cause was YAML parsing in .github/workflows/skill-docs.yml from unquoted run: scalars containing Run: .... The PR rewrites those steps as block scalars and adds a working actionlint workflow pinned to rhysd/actionlint@v1.7.11.

The upstream PR checks on garrytan/gstack are currently action_required at 0s, which looks like the standard forked-workflow approval gate. Once a maintainer approves the workflow run for this PR, the checks should execute normally.