feat: @project mentions with colored chips in markdown and editors

Add project mention system using project:// URI scheme with optional
color parameter. Mentions render as colored pill chips in markdown
bodies and the WYSIWYG editor. Autocomplete in editors shows both
agents and projects. Server extracts mentioned project IDs from issue
content and returns them in the issue detail response.

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

ui/src/components/CommentThread.tsx

@@ -45,6 +45,7 @@ interface CommentThreadProps {
   liveRunSlot?: React.ReactNode;
   enableReassign?: boolean;
   reassignOptions?: ReassignOption[];
+  mentions?: MentionOption[];
 }
 
 const CLOSED_STATUSES = new Set(["done", "cancelled"]);
@@ -110,6 +111,7 @@ export function CommentThread({
   liveRunSlot,
   enableReassign = false,
   reassignOptions = [],
+  mentions: providedMentions,
 }: CommentThreadProps) {
   const [body, setBody] = useState("");
   const [reopen, setReopen] = useState(true);
@@ -145,6 +147,7 @@ export function CommentThread({
 
   // Build mention options from agent map (exclude terminated agents)
   const mentions = useMemo<MentionOption[]>(() => {
+    if (providedMentions) return providedMentions;
     if (!agentMap) return [];
     return Array.from(agentMap.values())
       .filter((a) => a.status !== "terminated")
@@ -152,7 +155,7 @@ export function CommentThread({
         id: a.id,
         name: a.name,
       }));
-  }, [agentMap]);
+  }, [agentMap, providedMentions]);
 
   useEffect(() => {
     if (!draftKey) return;