feat(mention): add @mention plugin with async query and live-preview …

[asdasd123-w]

Summary

Introduces @floatboat/nexus-plugin-mention, a headless mention plugin for Nexus-Editor.

Motivation

• Roadmap item feat(toolbar): extend list toggle to multi-line selections #20 (shared comments / @mention) requires a foundational mention system.
• AI/LLM applications increasingly need @-agent references.
• Existing solutions lack AST-driven, round-trip-safe mention support in Markdown.

What's included

File	Purpose
src/index.ts	Public API: createMentionPlugin() factory
src/mention-types.ts	Strict TypeScript types
src/mention-ast.ts	remark plugin for [@name](id) ↔ mention node
src/mention-extension.ts	CM6 ViewPlugin: trigger detection, floating menu
src/mention-menu.ts	Headless DOM builder with full ARIA
src/mention-widget.ts	Live-preview chip widget
tests/*.test.ts	Vitest unit + integration tests
openspec/.../proposal.md	OpenSpec design doc

Features

• Trigger-based: type @ to open menu
• Async query: debounced with AbortController cancellation
• Keyboard navigation: ↑ ↓ Enter Esc Tab
• ARIA accessible: role="listbox", aria-selected, aria-activedescendant
• Headless: zero built-in CSS
• AST round-trip: [@name](id) ↔ mention mdast node
• Live preview: styled chips when cursor is away

Backwards Compatibility

Fully additive. No breaking changes.

AI Disclosure

Portions of this PR were generated with AI assistance and manually reviewed for correctness, style consistency, and test coverage.

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