feat(mobile): surface "Link an existing PR" in PR sidebar empty state (#5963)

* feat(mobile): surface "Link an existing PR" in PR sidebar empty state

The mobile link-PR building blocks (MobileLinkPrForm, linkMobilePr,
parseGitHubPrReference) existed and were tested, but had no entry point —
only unlink was wired up. Add a "Link an existing PR" action to the no-PR
empty state that opens MobileLinkPrForm and refetches the sidebar on
success, mirroring desktop's link flow (GitHub-scoped via worktree.set).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Polish mobile PR link empty state

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Jinwoo-H <jinwoo0825@gmail.com>

mobile/src/components/pr-sidebar/PrSidebarCreateEmptyState.tsx

@@ -1,12 +1,13 @@
 import { useEffect, useState } from 'react'
 import { ActivityIndicator, Pressable, Text, View } from 'react-native'
-import { GitPullRequestArrow, RefreshCw } from 'lucide-react-native'
+import { GitPullRequestArrow, Link2, RefreshCw } from 'lucide-react-native'
 import { colors } from '../../theme/mobile-theme'
 import type { RpcClient } from '../../transport/rpc-client'
 import { resolveMobilePrPrefill, type MobilePrPrefill } from '../../source-control/mobile-pr-create'
 import { fetchWorktreeLinkedPR } from '../../source-control/mobile-pr-link'
 import { openMobilePrUrl } from '../MobilePrComposeSheet'
 import { MobilePrComposeForm } from './MobilePrComposeForm'
+import { MobileLinkPrForm } from './MobileLinkPrForm'
 import { prCreateEmptyStateStyles as styles } from './pr-create-empty-state-styles'
 
 type Props = {
@@ -17,18 +18,18 @@ type Props = {
   onCreated: () => void
 }
 
-type Mode = 'choose' | 'create'
+type Mode = 'choose' | 'create' | 'link'
 
-// Empty state for a branch with no PR. Keep this scoped to desktop's no-PR
-// surface: create/refresh here; linked-PR edits belong outside this panel.
+// Empty state for a branch with no PR: create a new PR, or link an existing one
+// (the no-PR surface is the natural home for linking — desktop's link entry lives
+// on its PR card, but on mobile this is where a user lands with nothing linked).
 export function PrSidebarCreateEmptyState({ client, worktreeId, gitBranch, onCreated }: Props) {
   const [prefill, setPrefill] = useState<MobilePrPrefill | null>(null)
   const [mode, setMode] = useState<Mode>('choose')
   const [loading, setLoading] = useState(false)
   const [createWarning, setCreateWarning] = useState<string | null>(null)
   // A persisted linkedPR while the branch shows no PR means the linked PR could
-  // not be resolved. Mention it, but keep link editing out of this desktop-parity
-  // create surface.
+  // not be resolved. Mention it while still allowing the user to relink.
   const [orphanLinkedPR, setOrphanLinkedPR] = useState<number | null>(null)
 
   useEffect(() => {
@@ -103,6 +104,22 @@ export function PrSidebarCreateEmptyState({ client, worktreeId, gitBranch, onCre
     )
   }
 
+  if (mode === 'link') {
+    return (
+      <View style={styles.composerArea}>
+        <MobileLinkPrForm
+          client={client}
+          worktreeId={worktreeId}
+          onCancel={() => setMode('choose')}
+          onLinked={() => {
+            setMode('choose')
+            onCreated()
+          }}
+        />
+      </View>
+    )
+  }
+
   return (
     <View style={styles.section}>
       <View style={styles.header}>
@@ -148,6 +165,26 @@ export function PrSidebarCreateEmptyState({ client, worktreeId, gitBranch, onCre
               : 'The current branch is not linked to an open PR.'}
         </Text>
         {createWarning ? <Text style={styles.bodyText}>{createWarning}</Text> : null}
+        <Pressable
+          style={({ pressed }) => [
+            styles.linkButton,
+            !client && styles.linkButtonDisabled,
+            pressed && styles.linkButtonPressed
+          ]}
+          onPress={() => setMode('link')}
+          disabled={!client}
+          accessibilityRole="button"
+          accessibilityLabel="Link an existing pull request"
+          accessibilityState={{ disabled: !client }}
+          hitSlop={6}
+        >
+          <Link2
+            size={14}
+            color={client ? colors.textSecondary : colors.textMuted}
+            strokeWidth={2.2}
+          />
+          <Text style={styles.linkButtonText}>Link an existing PR</Text>
+        </Pressable>
       </View>
     </View>
   )

mobile/src/components/pr-sidebar/pr-create-empty-state-styles.ts

@@ -83,5 +83,25 @@ export const prCreateEmptyStateStyles = StyleSheet.create({
     borderBottomWidth: StyleSheet.hairlineWidth,
     borderBottomColor: colors.borderSubtle,
     padding: spacing.md
+  },
+  // Secondary link-an-existing-PR affordance, set apart from the body copy.
+  linkButton: {
+    marginTop: spacing.xs,
+    minHeight: 32,
+    alignSelf: 'flex-start',
+    flexDirection: 'row',
+    alignItems: 'center',
+    gap: spacing.xs
+  },
+  linkButtonDisabled: {
+    opacity: 0.5
+  },
+  linkButtonPressed: {
+    opacity: 0.6
+  },
+  linkButtonText: {
+    color: colors.textSecondary,
+    fontSize: typography.metaSize,
+    fontWeight: '600'
   }
 })