feat: add markdown heading fold/unfold in editor

[hoiyada7-maker]

Summary

• Adds section folding for ATX headings (# ## ### etc.) in the CodeMirror editor
• Custom foldService folds all content under a heading until the next equal-or-higher-level heading
• Gutter fold markers (▾/▸) appear on heading lines — click to fold/unfold
• Fold placeholder styled with accent color for visual clarity

Details

• foldGutter() + custom markdownHeadingFold service added to editor extensions
• foldKeymap registered: Ctrl+Shift+[ to fold, Ctrl+Shift+] to unfold
• Gutter markers shown at 50% opacity, full opacity on hover — unobtrusive but discoverable

Test plan

• Open a markdown file with multiple heading levels (#, ##, ###)
• Verify fold gutter markers appear on heading lines
• Click a marker to fold the section — content collapses to a placeholder
• Click the placeholder or marker again to unfold
• Test Ctrl+Shift+[ / Ctrl+Shift+] keyboard shortcuts

🤖 Generated with Claude Code