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Remark plugin that transforms inline `@marker:id` mention tokens in markdown text nodes into synthetic `link` mdast nodes using the `mention://` scheme, enabling downstream rendering as interactive chip components.

## Key Components

### `MENTION_REGEX`
Compiled regex matching `@<marker>:<id>` tokens with boundary detection. Captures an optional leading boundary character (`lead`), the marker (lowercase letters), and the id (alphanumeric plus `_.+/=-`, but not trailing `.` or `-`).

### `remarkMentionChips`
A [unified](https://github.com/unifiedjs/unified) `Plugin<[], Root>` that visits every `text` leaf in the mdast tree. For each match it:
1. Emits any preceding text as a `Text` node
2. Replaces the token with a `Link` node (`url: mention://<marker>:<id>`, children: raw token as fallback text)
3. Returns `[SKIP, index + parts.length]` to advance past the injected nodes without re-visiting them

## Usage Example

```typescript
import { unified } from 'unified'
import remarkParse from 'remark-parse'
import remarkRehype from 'remark-rehype'
import { remarkMentionChips } from './remark-mention-chips'

const processor = unified()
  .use(remarkParse)
  .use(remarkMentionChips)
  .use(remarkRehype)

// Input markdown:  "Check @device:64f0a1 and @kb:5 for details."
// Produces a link node: { type: 'link', url: 'mention://device:64f0a1', ... }
// The downstream <a> override detects the mention:// scheme and renders a chip.
const file = await processor.process('Check @device:64f0a1 and @kb:5 for details.')
```

## Notes

- Handles boundary safety: `user@host:1234` is **not** matched because `@` is preceded by a word character
- Parenthesised mentions like `(@device:64f0a1)` work correctly — the `(` is re-emitted as text
- Unresolved mentions (no matching `contextItems` entry) fall back to the raw `@marker:id` literal
- Companion to `remark-card-links`, which handles assistant-side `[card://type:id]` markers

**Source:** [`remark-mention-chips.ts`](https://github.com/flamingo-stack/openframe-oss-lib/blob/main/remark-mention-chips.ts)