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-6-mcp.md

@@ -0,0 +1,50 @@
+# Module 6: MCP Integration
+
+## What is MCP?
+
+- Model Context Protocol — a standard for connecting AI to external tools
+- Think of it as "USB ports for AI" — plug in any compatible tool
+- The GitHub MCP server is **built-in** (search repos, issues, PRs, actions)
+
+## Key commands
+
+| Command | What it does |
+|---------|-------------|
+| `/mcp` | List connected MCP servers |
+| `/mcp add <name> <command>` | Add a new MCP server |
+
+## Popular MCP servers
+
+- `@modelcontextprotocol/server-postgres` — Query PostgreSQL databases
+- `@modelcontextprotocol/server-sqlite` — Query SQLite databases
+- `@modelcontextprotocol/server-filesystem` — Access local files with permissions
+- `@modelcontextprotocol/server-memory` — Persistent knowledge graph
+- `@modelcontextprotocol/server-puppeteer` — Browser automation
+
+## Configuration
+
+| Level | File |
+|-------|------|
+| User | `~/.copilot/mcp-config.json` |
+| Project | `.github/mcp-config.json` |
+
+## Config file format
+
+```json
+{
+  "mcpServers": {
+    "my-server": {
+      "command": "npx",
+      "args": ["@modelcontextprotocol/server-postgres", "{{env.DATABASE_URL}}"],
+      "env": { "NODE_ENV": "development" }
+    }
+  }
+}
+```
+
+## Security best practices
+
+- Never put credentials directly in config files
+- Use environment variable references: `{{env.SECRET}}`
+- Review MCP server source before using
+- Only connect servers you actually need