Initial implementation of slider

Single-file markdown slideshow for the terminal. Parses slides split by
`---`, renders each with glow, and navigates with n/p/q or arrow keys.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

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+# slider
+
+Terminal markdown slideshow. Write slides in a single `.md` file, split them with `---`, and navigate with keyboard shortcuts.
+
+## Requirements
+
+- [glow](https://github.com/charmbracelet/glow) v2+
+
+## Usage
+
+```bash
+./bin/slider presentation.md
+```
+
+## Controls
+
+| Key | Action |
+|-----|--------|
+| `n` / right arrow | Next slide |
+| `p` / left arrow | Previous slide |
+| `q` | Quit |
+
+## Writing slides
+
+Separate slides with a `---` on its own line:
+
+```markdown
+# Slide one
+
+Some content here.
+
+---
+
+# Slide two
+
+More content.
+```
+
+See `examples/presentation.md` for a full example.