fix: rewrite plugin.json paths to folders during publish

During the staged→main publish, after materializing files into plugin
directories, rewrite each plugin.json to replace individual file paths
with folder references so consumers on main get directory-level entries.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

eng/materialize-plugins.mjs

@@ -145,6 +145,30 @@ function materializePlugins() {
       }
     }
 
+    // Rewrite plugin.json to use folder paths instead of individual file paths.
+    // On staged, paths like ./agents/foo.md point to individual source files.
+    // On main, after materialization, we only need the containing directory.
+    const rewritten = { ...metadata };
+    let changed = false;
+
+    for (const field of ["agents", "commands"]) {
+      if (Array.isArray(rewritten[field]) && rewritten[field].length > 0) {
+        const dirs = [...new Set(rewritten[field].map(p => path.dirname(p)))];
+        rewritten[field] = dirs;
+        changed = true;
+      }
+    }
+
+    if (Array.isArray(rewritten.skills) && rewritten.skills.length > 0) {
+      // Skills are already folder refs (./skills/name/); strip trailing slash
+      rewritten.skills = rewritten.skills.map(p => p.replace(/\/$/, ""));
+      changed = true;
+    }
+
+    if (changed) {
+      fs.writeFileSync(pluginJsonPath, JSON.stringify(rewritten, null, 2) + "\n", "utf8");
+    }
+
     const counts = [];
     if (metadata.agents?.length) counts.push(`${metadata.agents.length} agents`);
     if (metadata.commands?.length) counts.push(`${metadata.commands.length} commands`);