feat(P5C.T3): recompute message_mentions on edit

After the ciphertext UPDATE succeeds in editEncryptedMessage, delete the
existing message_mentions rows for that message then re-resolve and
re-insert from the new plaintext — matching the best-effort pattern used
by insertMessage. No signature change needed; conversationId and
newPlaintext were already present in EditMessageParams.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/shared/src/chat/messages.ts

@@ -226,6 +226,33 @@ export async function editEncryptedMessage(
     } as never)
     .eq('id', params.messageId);
   if (error) throw error;
+
+  // Recompute mentions: an edit can add or remove @-tokens. Drop old rows
+  // then re-insert from the new plaintext (best-effort, same as insertMessage).
+  try {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const { error: delErr } = await (params.client as any)
+      .from('message_mentions')
+      .delete()
+      .eq('message_id', params.messageId);
+    if (delErr) throw delErr;
+
+    const mentionUsernames = parseMentionUsernames(params.newPlaintext);
+    if (mentionUsernames.length > 0) {
+      const resolver = makeMentionResolver(params.client);
+      const resolved = await resolver.resolveUsernames(params.conversationId, mentionUsernames);
+      if (resolved.size > 0) {
+        await insertMentions(
+          params.client,
+          params.messageId,
+          params.conversationId,
+          [...resolved.values()],
+        );
+      }
+    }
+  } catch (err) {
+    console.warn('mention recompute on edit failed', err);
+  }
 }
 
 export async function softDeleteMessage(