feat: add cli-mastery skill — interactive Copilot CLI training (#915)

* feat: add cli-mastery skill — interactive Copilot CLI training

Adds cli-mastery, an interactive training system for the GitHub Copilot CLI.
8 modules covering slash commands, keyboard shortcuts, modes, agents, skills,
MCP, configuration, and advanced techniques. Includes scenario challenges,
a final exam, XP/leveling system, and SQL-based progress tracking.

Source: https://github.com/DUBSOpenHub/copilot-cli-mastery (MIT)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: address review feedback on frontmatter and consistency

- Switch description from folded block scalar (>) to single-quoted string
  per AGENTS.md documented format
- Fix Module 7 heading: backtick-wrap @ separately from 'file mentions'
  to avoid implying '@ file mentions' is a literal command
- Fix Final Exam Q6: change '@ + filename' to '@filename' with example
  to match the @src/auth.ts syntax taught in modules
- Fix Final Exam Q7: add GEMINI.md to match Module 7 precedence list

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* fix: rename curriculum/ to references/ per agentskills.io spec

Addresses review feedback from @aaronpowell on PR #915.
The Agent Skills specification defines references/ as the standard
directory for supplementary documentation that agents read on demand.

- Renamed skills/cli-mastery/curriculum/ → references/
- Updated all path references in SKILL.md
- Updated asset paths in docs/README.skills.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: DUBSOpenHub <DUBSOpenHub@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

skills/cli-mastery/references/final-exam.md

@@ -0,0 +1,24 @@
+# Final Exam
+
+Present a 10-question comprehensive exam using `ask_user` with 4 choices each. Require 80%+ to pass. Vary the selection each time.
+
+## Question Bank
+
+1. Which command initializes Copilot CLI in a new project? → `/init`
+2. What shortcut cycles through modes? → `Shift+Tab`
+3. Where are repo-level custom agents stored? → `.github/agents/*.md`
+4. What does MCP stand for? → Model Context Protocol
+5. Which agent is safe to run in parallel? → `explore`
+6. How do you add a file to AI context? → `@filename` (e.g. `@src/auth.ts`)
+7. What file has the highest instruction precedence? → `CLAUDE.md` / `GEMINI.md` / `AGENTS.md` (git root + cwd)
+8. Which command compresses conversation history? → `/compact`
+9. Where is MCP configured at project level? → `.github/mcp-config.json`
+10. What does `--yolo` do? → Same as `--allow-all` (skip all confirmations)
+11. What does `/research` do? → Run a deep research investigation with sources
+12. Which shortcut opens input in $EDITOR? → `Ctrl+G`
+13. What does `/reset-allowed-tools` do? → Re-enables confirmation prompts
+14. Which command copies the last AI response to your clipboard? → `/copy`
+15. What does `/compact` do? → Summarizes conversation to free context
+
+On pass (80%+): Award "CLI Wizard" title, congratulate enthusiastically!
+On fail: Show which they got wrong, encourage retry.