#2973 Replace markdown pane with STTextView

Replaces the markdown pane implementation with STTextView.

What changed:

• swaps the package dependency from MarkdownUI to STTextView
• renders markdown into a read-only STTextView surface
• keeps Ghostty font family, font size, and background/foreground colors
• enables line numbers in the markdown pane
• keeps link attributes and opens links on cmd-click
• adds focused unit coverage for renderer styling, theme application, and cmd-click link policy

Verification:

• xcodebuild -project GhosttyTabs.xcodeproj -scheme cmux-unit -destination 'platform=macOS' -derivedDataPath /tmp/cmux-sttext-unit test -only-testing:cmuxTests/MarkdownPanelTextViewTests
• ./scripts/reload.sh --tag sttext

---

Summary by cubic

Replaces the markdown pane with STTextView for native text rendering while keeping the Ghostty look and feel. Adds line numbers and cmd-click to open links, and removes MarkdownUI.

• New Features

  • Read-only STTextView markdown surface with Ghostty font, size, and colors.
  • Line numbers enabled; links keep attributes and open on cmd-click.
  • Preserves scroll position on content/theme changes; adds focused tests for styling, theming, and link policy.

• Dependencies

  • Swap MarkdownUI → STTextView (adds CoreTextSwift and STTextKitPlus).
  • Update Xcode project and Package.resolved.

Summary by CodeRabbit

• Refactor

  • Updated markdown panel rendering with improved text display component.
  • Links in markdown panels now require Command modifier to open.

• Tests

  • Added comprehensive test coverage for markdown panel functionality.