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/components/markdown-text.ios.tsx

@@ -8,10 +8,11 @@ interface MarkdownTextSpanProps {
   children: ReactNode;
   // Links route through this span too (see assistant-file-links/link.tsx). A
   // plain <Text> nested in the paragraph UITextView is dropped, so the link
-  // must be a UITextView span to be visible. onPress is forwarded best-effort:
-  // react-native-uitextview nulls onPress on the root native view, so reliable
-  // tap-to-open is still tracked by #21 — but visible+selectable text beats an
-  // invisible link.
+  // must be a UITextView span to be visible. onPress is wired onto the leaf
+  // string children here: react-native-uitextview attaches it to the
+  // RNUITextViewChild nodes it builds from string content, which the native tap
+  // recognizer dispatches to. The link's handler reaches these leaf spans via
+  // AssistantLinkPressProvider (see assistant-file-links/link-press-context).
   onPress?: TextProps["onPress"];
   accessibilityRole?: TextProps["accessibilityRole"];
 }