feat: add collapsible sections plugin

[hung6895]

Summary

• Adds a collapsibleSectionsPlugin that enables collapsing/expanding document sections by heading level
• Click the left 24px of any heading to toggle collapse — content under that heading (until the next heading of same or higher level) is hidden
• Exposes toggleAllSections$ and toggleHeadingCollapse$ signals for programmatic control
• Includes a Ladle story (CollapsibleSections) with Collapse All / Expand All controls for testing

Test plan

• Run npm run typecheck — passes
• Run npm run lint — passes
• Run npm run test:once — existing tests pass
• Open Ladle story and verify:
  • Chevron indicators appear on heading hover
  • Clicking left 24px of a heading collapses/expands its section
  • Clicking the right side of a heading still allows text editing
  • Collapsing an H1 hides all nested H2/H3 content
  • Expanding an H1 preserves nested heading collapse state
  • "Collapse All" / "Expand All" buttons work
  • Markdown output is unchanged after collapse/expand (transient UI-only state)
  • Adding/removing headings updates collapse UI correctly

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