fix(app): make markdown links tappable on iOS (#1334)

* fix(app): make markdown links tappable on iOS

A markdown link's label renders as nested React elements
(link > textgroup > text -> MarkdownInheritedText spans). The link's
onPress was placed on the wrapping UITextView span, but
react-native-uitextview only attaches onPress to the *string* children
it converts into tappable RNUITextViewChild nodes - element children
pass through untouched, so the handler never reached a node the native
tap recognizer could fire. Link text stayed visible and selectable but
dead to taps; web links never opened.

Thread the link's press handler down to the leaf text spans via a new
AssistantLinkPressProvider so it lands on the real tappable string
nodes. MarkdownInheritedText consumes the context and forwards onPress
to its MarkdownTextSpan; null outside a link, so ordinary text is
unaffected. Gated to iOS - Android forwards onPress through nested
<Text> already, and web uses the <a> path.

* refactor(app): export useAssistantLinkPress from module index

Import it through the assistant-file-links public surface instead of
reaching into the internal link-press-context file, matching the rest of
message.tsx. Addresses Greptile review feedback on #1334.

---------

Co-authored-by: Kevin Aspesi <kevinaspesi@Kevins-Mac-mini.local>

packages/app/src/assistant-file-links/index.ts

@@ -3,6 +3,7 @@ export {
   AssistantMarkdownCodeLink,
   AssistantMarkdownLink,
 } from "./link";
+export { type AssistantLinkPress, useAssistantLinkPress } from "./link-press-context";
 export {
   classifyAssistantFileLink,
   normalizeInlinePathTarget,