fix: patch name: in SKILL.md frontmatter for prefix mode (#620, #578)

./setup --prefix creates gstack-* symlinks but SKILL.md still says
name: qa, so Claude Code ignores the prefix. Now:
- New bin/gstack-patch-names shared helper patches name: field via sed
- setup calls it after link_claude_skill_dirs
- gstack-relink calls it after symlink loop
- gen-skill-docs.ts prints warning when skill_prefix is true

Edge cases: gstack-upgrade not double-prefixed, root gstack skill
never prefixed, prefix removal restores original names, SKILL.md
without frontmatter is a safe no-op.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/gen-skill-docs.ts

@@ -472,3 +472,16 @@ if (failures.length > 0 && HOST_ARG_VAL === 'all') {
   if (failures.some(f => f.host === 'claude')) process.exit(1);
 }
 // Single host dry-run failure already handled above
+
+// After all hosts processed, warn if prefix patches may need re-applying
+if (!DRY_RUN) {
+  try {
+    const configPath = path.join(process.env.HOME || '', '.gstack', 'config.yaml');
+    if (fs.existsSync(configPath)) {
+      const config = fs.readFileSync(configPath, 'utf-8');
+      if (/^skill_prefix:\s*true/m.test(config)) {
+        console.log('\nNote: skill_prefix is true. Run gstack-relink to re-apply name: patches.');
+      }
+    }
+  } catch { /* non-fatal */ }
+}