fix(coder): parse YAML block-scalar descriptions in slash command discovery

Most plugin/han SKILL.md and command files write `description:` as a folded
block scalar (`>` / `|`) with the text on the following indented lines. The
old single-line frontmatter reader captured the literal `>`, so the slash
menu showed garbage/blank descriptions for nearly all of them. frontmatterField
now collapses folded blocks (join with spaces) and preserves literal blocks.

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

apps/coder/src/services/claude-command-discovery.ts

@@ -12,12 +12,48 @@ import { homedir } from 'node:os';
 import { join } from 'node:path';
 import type { AgentCommand } from './provider-types.js';
 
-/** Minimal frontmatter reader — single-line `key: value` between `---` fences. */
+/**
+ * Frontmatter reader between `---` fences. Handles single-line `key: value`
+ * AND YAML block scalars (`key: >` folded / `key: |` literal) whose value
+ * spans the following more-indented lines — the shape most plugin SKILL.md
+ * descriptions use (`description: >`).
+ */
 function frontmatterField(content: string, field: string): string | undefined {
   const block = content.match(/^---\r?\n([\s\S]*?)\r?\n---/);
   if (!block?.[1]) return undefined;
-  const m = block[1].match(new RegExp(`^${field}:\\s*(.+)$`, 'm'));
+  const lines = block[1].split(/\r?\n/);
-  return m?.[1]?.trim().replace(/^["']|["']$/g, '') || undefined;
+  const keyRe = new RegExp(`^(\\s*)${field}:\\s*(.*)$`);
+  for (let i = 0; i < lines.length; i++) {
+    const m = lines[i]?.match(keyRe);
+    if (!m) continue;
+    const keyIndent = (m[1] ?? '').length;
+    const inline = (m[2] ?? '').trim();
+    // Block scalar: `>` (folded) or `|` (literal), optional chomping `+`/`-`.
+    if (/^[>|][+-]?$/.test(inline)) {
+      const folded = inline[0] === '>';
+      const body: string[] = [];
+      for (let j = i + 1; j < lines.length; j++) {
+        const line = lines[j] ?? '';
+        if (line.trim() === '') {
+          body.push('');
+          continue;
+        }
+        const indent = line.length - line.trimStart().length;
+        if (indent <= keyIndent) break; // dedent ends the block
+        body.push(line.slice(keyIndent + 1));
+      }
+      const joined = folded
+        ? body
+            .map((l) => l.trim())
+            .join(' ')
+            .replace(/\s+/g, ' ')
+            .trim()
+        : body.join('\n').replace(/\n+$/, '');
+      return joined || undefined;
+    }
+    return inline.replace(/^["']|["']$/g, '').trim() || undefined;
+  }
+  return undefined;
 }
 
 function readCommandDir(dir: string): AgentCommand[] {