feat(explainer): wire export_bundle into the publish stage

Make the new PUBLISH-tier tool usable rather than dead code: add export_bundle
to the animated-explainer publish stage's tools_available, and update the
explainer publish-director skill Step 5 to call export_bundle for packaging
(passing the prepared metadata) instead of hand-creating the export tree. The
documented export layout and the no-upload caveat are kept in sync with the
tool. Other pipelines' publish stages can adopt it the same way.

pipeline_defs/animated-explainer.yaml

@@ -254,7 +254,7 @@ stages:
       - proposal_packet
     produces:
       - publish_log
-    tools_available: []
+    tools_available: [export_bundle]
     checkpoint_required: true
     human_approval_default: true
     review_focus:

skills/pipelines/explainer/publish-director.md

@@ -85,22 +85,34 @@ Each chapter maps to a script section's `start_seconds`.
 
 ### Step 5: Package Export
 
-Create the export directory structure:
+Use the `export_bundle` tool (capability `publish`) to do the packaging
+deterministically — pass it the final `video_path` (from `render_report`), the
+`title`, and the metadata you prepared (`description`, `tags`, `hashtags`,
+`chapters`, optional `subtitles_path` and `thumbnail_path`/`thumbnail_concept`).
+It lays out the export directory, writes the metadata files, and returns a
+schema-valid `publish_log` (`status: "exported"`) in `data["publish_log"]` that
+you persist as the stage artifact.
+
+It produces this structure:
 
 ```
 exports/
   <project_name>/
     video/
-      output.mp4            # Final rendered video
+      output.mp4            # Final rendered video (subtitles.srt alongside if provided)
     metadata/
       metadata.json         # All SEO metadata
       chapters.txt          # Chapter markers
-      description.txt       # Ready-to-paste description
+      description.txt       # Ready-to-paste description (+ chapters)
       tags.txt              # One tag per line
     thumbnails/
-      concept.json          # Thumbnail concept (or generated image)
+      concept.json          # Thumbnail concept (or the copied thumbnail image)
 ```
 
+`export_bundle` is a local, offline packager — it does not upload. A networked
+publisher (e.g. a YouTube uploader) would be a separate `publish`-capability
+provider.
+
 ### Step 6: Build Publish Log
 
 ```json