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-2-keyboard-shortcuts.md

@@ -0,0 +1,38 @@
+# Module 2: Keyboard Shortcuts
+
+## Navigation & Editing
+
+| Shortcut | Action |
+|----------|--------|
+| `@` | Mention files — include their contents as context |
+| `Ctrl+S` | Submit prompt while preserving input text |
+| `Shift+Tab` | Cycle modes: Interactive → Plan |
+| `Ctrl+T` | Toggle model reasoning display |
+| `Ctrl+O` | Expand recent timeline (when no input) |
+| `Ctrl+E` | Expand all timeline (when no input) / move to end of line (when typing) |
+| `↑` `↓` | Navigate command history |
+| `!` | Execute shell command directly (bypass AI) |
+| `Esc` | Cancel current operation |
+| `Ctrl+C` | Cancel operation / clear input / exit |
+| `Ctrl+D` | Shutdown session |
+| `Ctrl+L` | Clear the screen |
+| `Ctrl+G` | Edit prompt in external editor ($EDITOR) |
+
+## Line Editing
+
+| Shortcut | Action |
+|----------|--------|
+| `Ctrl+A` | Move to beginning of line |
+| `Ctrl+H` | Delete previous character |
+| `Ctrl+W` | Delete previous word |
+| `Ctrl+U` | Delete from cursor to beginning of line |
+| `Ctrl+K` | Delete from cursor to end of line |
+| `Meta+←` `Meta+→` | Move cursor by word |
+
+## Pro tips to teach
+
+- `@` is THE most important shortcut — it's how you give precise context
+- `!git status` runs git directly without AI processing
+- `Shift+Tab` into Plan mode BEFORE complex tasks
+- `Ctrl+G` opens your $EDITOR for long prompts — game changer
+- `Ctrl+S` lets you iterate on a prompt without retyping