blendtutor-course skill: author courses, lessons, evals, and Quarto/site output

[mcmullarkey]

Summary

Adds a project skill, .claude/skills/blendtutor-course/SKILL.md, that walks an author from source material to a published blendtutor course:

1. Scaffold with blendtutor init and blendtutor new lesson.
2. Write lessons that work in webR/Pyodide: inline data, checks that assert results (R stopifnot, Python assert), a solution that passes them, bullet hints/gotchas, and success_criteria for what checks cannot see.
3. Write a minimal eval: four cases (canonical correct, alternative correct, near-miss that passes checks but misses the point, check-caught mistake).
4. Verify locally without paid calls: validate, list, plus copy-paste R and Python scripts that run the solution and every eval case against the checks. Paid blendtutor eval only after confirming with the user.
5. Publish: export-quarto snippet / --document / --key-page with the Quarto setup gotchas (install next to _quarto.yml, filter in one place, commit _extensions/, no file://), or blendtutor build --target webr|pyodide, with the --embed-key risk called out.

.gitignore now ignores .claude/* but shares .claude/skills/, so local Claude settings and plans stay out of git.

Test plan

• The Python verification script, extracted verbatim from SKILL.md, run against a real Python lesson + eval: solution and correct cases pass, near-miss passes checks, check-caught case fails
• The R verification script, extracted verbatim from SKILL.md, run against examples/write-less-code-r/01_seed_data.yaml + its eval: solution and correct cases pass, incorrect cases fail; no temp files left behind
• CLI commands and flags in the skill match --help for init, new lesson, validate, list, run, eval, build, export-quarto
• git check-ignore: SKILL.md tracked; .claude/settings.local.json and .claude/plans/ still ignored
• Invoke the skill end to end in a fresh Claude Code session on a new course