fix: align the preview workflow with ci and release

Three reusable workflows take the same layout inputs and all three defaulted
differently: preview left the plugin roots unset and still pointed at a
marketplace manifest. One layout described three ways is how a repository
passes one check and fails another, which is the failure this series started
from.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01MgsFmepr9FxbFkax5Y2h6z

.github/workflows/agent-marketplace-preview.yaml

@@ -12,7 +12,7 @@ on:
         description: Claude marketplace manifest relative to the repository root.
         required: false
         type: string
-        default: .claude-plugin/marketplace.json
+        default: ""
       claude_plugin_file:
         description: Claude plugin manifest relative to the repository root.
         required: false
@@ -22,10 +22,12 @@ on:
         description: Codex plugin root relative to the repository root.
         required: false
         type: string
+        default: "."
       cursor_plugin_root:
         description: Cursor plugin root relative to the repository root.
         required: false
         type: string
+        default: ""
       package_paths:
         description: Space-separated paths to package into the preview archive.
         required: false