feat(web): a prose mention of a produced file opens it

The chatFileMentions service (provided by ui-deliverables beside its
turn-tail entry, reached via ctx.get) resolves inline-code tokens in the
closing message against the turn's produced locations: exact path or
unique basename links, ambiguity and unknowns stay inert. MarkdownText
gains the optional fileMentions seam — settled renders only, never
inside anchors.

packages/client/ui-primitives/src/index.ts

@@ -40,7 +40,7 @@ export { CodeBlock } from './markdown/CodeBlock.tsx'
 export type { CodeBlockProps } from './markdown/CodeBlock.tsx'
 export { JsonBlock } from './markdown/JsonBlock.tsx'
 export { MarkdownText } from './markdown/MarkdownText.tsx'
-export type { MarkdownCodeLabels } from './markdown/MarkdownText.tsx'
+export type { MarkdownCodeLabels, MarkdownFileMentions } from './markdown/MarkdownText.tsx'
 export { MessageText } from './markdown/MessageText.tsx'
 export { extractMarkdownPlainText } from './markdown/plain-text.ts'
 export type { MarkdownPlainTextMode, MarkdownPlainTextOptions } from './markdown/plain-text.ts'