Builds on the merged #5774 and targets develop. Part 2/4 of GitHub Alerts support, split from #5770. Related to #3764 and #1240.

Summary

Connect the format support from #5774 to Muya's live editor. The option remains off by default; the desktop preference follows in Part 3 and menus follow in #5771.

• Register the dedicated github-alert block and share quote-container structure with ordinary blockquotes.
• Handle marker + Enter, empty-paragraph exit and Backspace through the existing paragraph dispatch, unwrapping inner blocks first.
• Preserve the existing direct state-loading API; desktop documents continue to enter through Markdown parsing.
• Add shared local conversion commands that preserve adjacent nodes and selection, with ordinary undo/redo behavior.
• Support rich HTML paste, partial/full copy and cut, nested-container demotion and Markdown export.
• Add editor styling, live parser-option forwarding and integration tests. Setting changes follow the existing reparse/render path without special selection or history migration.

Validation

• Full Muya unit suite: 2,693 passed.
• Full desktop/Muya type check, Muya lint, circular dependency check and build:unpack passed.
• Full standalone Chromium E2E: 483 passed.

Tests use an isolated worktree. Temporary local dependency-access configuration is not committed.

Review order

#5774 (syntax/conversion) → this PR (editor behavior) → #5776 (desktop integration) → #5771 (menus).

Merge each predecessor first, then retarget its immediate follow-up to develop. The base branch keeps this diff limited to editor integration and its tests.