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/InlineEditor.tsx

@@ -2,7 +2,7 @@ import { useState, useRef, useEffect, useCallback } from "react";
 import { cn } from "../lib/utils";
 import { Button } from "@/components/ui/button";
 import { MarkdownBody } from "./MarkdownBody";
-import { MarkdownEditor } from "./MarkdownEditor";
+import { MarkdownEditor, type MentionOption } from "./MarkdownEditor";
 
 interface InlineEditorProps {
   value: string;
@@ -12,6 +12,7 @@ interface InlineEditorProps {
   placeholder?: string;
   multiline?: boolean;
   imageUploadHandler?: (file: File) => Promise<string>;
+  mentions?: MentionOption[];
 }
 
 /** Shared padding so display and edit modes occupy the exact same box. */
@@ -25,6 +26,7 @@ export function InlineEditor({
   placeholder = "Click to edit...",
   multiline = false,
   imageUploadHandler,
+  mentions,
 }: InlineEditorProps) {
   const [editing, setEditing] = useState(false);
   const [draft, setDraft] = useState(value);
@@ -81,6 +83,7 @@ export function InlineEditor({
             placeholder={placeholder}
             contentClassName={className}
             imageUploadHandler={imageUploadHandler}
+            mentions={mentions}
             onSubmit={commit}
           />
           <div className="flex items-center justify-end gap-2">