feat(editor): highlight inbound citation fragments

[horacioh]

Summary

• Add editor decorations for inbound ranged citation fragments, including overlap-aware highlight styling and hover states.
• Wire citation fragment data into document pages with a menu toggle to show or hide highlights.
• Support click behavior for citation highlights: single citations navigate directly, while overlapping citations show a popover with document or comment context.
• Keep citation highlight clicks disabled while editing and split highlights around inserted text so new content is not incorrectly included.
• Add tests for overlapping decorations, click payloads, disabled interactivity, and inserted-text behavior.
• Add a project document for fragment citation highlights and a write-project-document agent skill.

[horacioh]

@GaboHBeaumont here's the annotation highlights code