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/module-5-skills.md

@@ -0,0 +1,33 @@
+# Module 5: Skills System
+
+## What are skills?
+
+- Specialized capability packages the AI can invoke
+- Think of them as "expert modes" with domain-specific knowledge
+- Managed via `/skills` command
+
+## Skill locations
+
+| Level | Location |
+|-------|----------|
+| User | `~/.copilot/skills/<name>/SKILL.md` |
+| Repo | `.github/skills/<name>/SKILL.md` |
+| Org | Shared via org-level config |
+
+## Creating a custom skill
+
+1. Create the directory: `mkdir -p ~/.copilot/skills/my-skill/`
+2. Create `SKILL.md` with YAML frontmatter (`name`, `description`, optional `tools`)
+3. Write detailed instructions for the AI's behavior
+4. Verify with `/skills`
+
+## Skill design best practices
+
+- **Clear description** — helps the AI match tasks to your skill automatically
+- **Focused scope** — each skill should do ONE thing well
+- **Include instructions** — specify exactly how the skill should operate
+- **Test thoroughly** — use `/skills` to verify, then invoke and check results
+
+## Auto-matching
+
+When you describe a task, the AI checks if any skill matches and suggests using it.