feat(mentions): support @mentioning issues + server-side auto-expansion (#242)

* feat(mentions): support @mentioning issues in comments

- Extend MentionItem type to include "issue" alongside "member"/"agent"
- Add issue search (by identifier and title) to mention suggestion dropdown
- Render issue mentions with CircleDot icon in autocomplete popup
- Issue mentions serialize as [MUL-117 Title](mention://issue/id) (no @ prefix)
- Markdown renderer shows issue mentions as clickable links to /issues/:id
- Backend mentionRe regex updated to match issue mention type

* feat(mentions): auto-expand issue identifiers and add mention format to agent instructions

1. Path A — CLAUDE.md template (runtime_config.go):
   Add a "## Mentions" section teaching agents the mention serialization
   format for issues, members, and agents. All agents automatically
   receive this via the auto-generated CLAUDE.md.

2. Approach 2 — Server-side auto-conversion (internal/mention/):
   New ExpandIssueIdentifiers() utility that scans comment content for
   bare issue identifiers (e.g. MUL-117) and replaces them with
   [MUL-117](mention://issue/<uuid>) mention links. Skips code blocks,
   inline code, and existing markdown links. Integrated into both:
   - handler.CreateComment (HTTP API path)
   - service.createAgentComment (agent task output path)

apps/web/components/common/rich-text-editor.tsx

@@ -63,6 +63,8 @@ const MentionExtension = Mention.configure({
   suggestion: createMentionSuggestion(),
 }).extend({
   renderHTML({ node, HTMLAttributes }) {
+    const type = node.attrs.type ?? "member";
+    const prefix = type === "issue" ? "" : "@";
     return [
       "span",
       mergeAttributes(
@@ -74,7 +76,7 @@ const MentionExtension = Mention.configure({
           "data-mention-id": node.attrs.id,
         },
       ),
-      `@${node.attrs.label ?? node.attrs.id}`,
+      `${prefix}${node.attrs.label ?? node.attrs.id}`,
     ];
   },
   addAttributes() {
@@ -89,15 +91,16 @@ const MentionExtension = Mention.configure({
     };
   },
   // @tiptap/markdown: custom tokenizer to parse [@Label](mention://type/id)
+  // and [Label](mention://issue/id) (issue mentions have no @ prefix)
   markdownTokenizer: {
     name: "mention",
     level: "inline" as const,
     start(src: string) {
-      return src.search(/\[@[^\]]+\]\(mention:\/\//);
+      return src.search(/\[@?[^\]]+\]\(mention:\/\//);
     },
     tokenize(src: string) {
       const match = src.match(
-        /^\[@([^\]]+)\]\(mention:\/\/(\w+)\/([^)]+)\)/,
+        /^\[@?([^\]]+)\]\(mention:\/\/(\w+)\/([^)]+)\)/,
       );
       if (!match) return undefined;
       return {
@@ -114,7 +117,8 @@ const MentionExtension = Mention.configure({
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
   renderMarkdown: (node: any) => {
     const { id, label, type = "member" } = node.attrs || {};
-    return `[@${label ?? id}](mention://${type}/${id})`;
+    const prefix = type === "issue" ? "" : "@";
+    return `[${prefix}${label ?? id}](mention://${type}/${id})`;
   },
 });