Replace monokai with GitHub-like Rouge syntax highlighting for consistent code fence rendering

[Copilot]

This PR addresses inconsistent syntax highlighting where some code blocks (notably bash) rendered poorly compared to others (e.g., powershell) due to a mismatch between the dark monokai theme and light Poole base styles.

Problem

The existing setup used:

• Dark monokai.css syntax theme (#272822 background)
• Light Poole pre styles (#f9f9f9 background)
• This created color contrast mismatches for certain lexers, making bash highlighting appear inconsistent

Solution

Replaced the dark monokai theme with Rouge's built-in GitHub theme:

1. Generated GitHub Rouge CSS: Used rougify style github > public/css/github-syntax.css
2. Updated CSS reference: Changed _includes/head.html to load the new GitHub theme instead of monokai
3. Preserved functionality: Maintained existing scrollability rules in custom.scss

Benefits

• Consistent highlighting across all languages (bash, powershell, yaml, json, javascript, python, etc.)
• GitHub-like appearance with proper light theme colors and contrast
• No more workarounds - no need to mislabel code blocks for better highlighting
• Seamless integration - GitHub theme's #f8f8f8 background harmonizes with existing Poole styles

Visual Result

The screenshot demonstrates consistent, professional syntax highlighting across multiple languages with proper contrast and readability on the light background.

Technical Details

• Uses Rouge's official GitHub theme for reliability and maintenance
• Preserves horizontal scrolling for long code lines
• Compatible with existing Jekyll + kramdown + Rouge setup
• Minimal changes: only 2 files modified

Fixes #7.

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