fix: correct skill-only .skill packaging structure in Step 6

skills/cowork-project/references/mode-document.md

@@ -174,10 +174,11 @@ Both destinations are required. Confirm both are written before proceeding.
 
 ---
 
-## Step 6 — Package the .plugin File (skip for skill-only with no MCP server)
+## Step 6 — Package the .skill / .plugin File
 
-If the plugin has an MCP server component (has `.mcp.json` and a `server/` directory),
-build a fresh `.plugin` package from the downloaded repo archive:
+### For plugins with an MCP server (has `.mcp.json` and `server/` directory)
+
+Build a `.plugin` package from the repo. No structural constraints on top-level folders:
 
 ```bash
 cd /tmp/plugin-repo/[repo-folder-name]
@@ -189,24 +190,53 @@ zip -r /tmp/[plugin-name]-v[version].plugin \
 # Add any additional skills/ subdirectories if multiple skills exist
 ```
 
-For skill-only plugins (no `server/` directory, no `.mcp.json`), package only the skill files:
+### For skill-only packages (no `server/` directory, no `.mcp.json`)
+
+**Critical structure requirement:** The zip must contain **exactly one top-level folder**
+with `SKILL.md` directly inside it. Do NOT zip `.claude-plugin/` and `skills/` as separate
+top-level entries — the upload validator will reject it with "Zip must contain exactly one
+top-level folder" and "SKILL.md must be in the top-level folder, not nested deeper."
 
 ```bash
-cd /tmp/plugin-repo/[repo-folder-name]
-zip -r /tmp/[plugin-name]-v[version].plugin \
-  .claude-plugin/plugin.json \
+# Stage into a clean directory with one top-level folder named after the skill
+mkdir -p /tmp/[skill-name]-pkg/[skill-name]/
+cp skills/[skill-name]/SKILL.md /tmp/[skill-name]-pkg/[skill-name]/SKILL.md
+# Copy any subdirectories (references/, assets/, etc.) if present
+cp -r skills/[skill-name]/references/ /tmp/[skill-name]-pkg/[skill-name]/references/
+
+# Zip from the staging dir — SKILL.md ends up at root of the single top-level folder
+cd /tmp/[skill-name]-pkg
+zip -r /tmp/[skill-name]-v[version].skill [skill-name]/
+```
+
+**Verify before uploading** — contents must look like this:
+```
+[skill-name]/
+  SKILL.md          ← directly here, NOT nested
+  references/       ← optional
+    *.md
+```
+
+NOT like this (will be rejected by the upload validator):
+```
+.claude-plugin/
+  plugin.json
 skills/
-# Include all files under skills/ recursively
+  [skill-name]/
+    SKILL.md
 ```
 
 **Upload to Drive:**
 Use `create_drive_file` with:
-- `file_name`: `[plugin-name]-v[version].plugin`
+- `file_name`: `[skill-name]-v[version].skill`
 - `folder_id`: the CW-XXX folder ID from Step 4
 - `mime_type`: `application/zip`
-- `fileUrl`: `file:///[outputs-path]/[plugin-name]-v[version].plugin`
+- `fileUrl`: `file:///[outputs-path]/[skill-name]-v[version].skill`
 
-If an older `.plugin` file exists in the folder, trash it first:
+Note: The Drive API server runs on the host, not the sandbox. Copy the file from `/tmp`
+to the outputs directory first, then reference it via the host path (not the VM `/tmp` path).
+
+If an older `.skill` or `.plugin` file exists in the folder, trash it first:
 - Use `search_drive_files` to find it by name pattern in the folder
 - Use `update_drive_file` with `trashed: true`
 
@@ -277,7 +307,7 @@ Before writing the registry row, confirm:
 
 - [ ] README.md is in the Drive folder (check via `search_drive_files` or confirm upload ID)
 - [ ] Wiki doc is in the Drive folder (confirm parent folder ID matches)
-- [ ] .plugin file is in the Drive folder (or note "skill-only, no .plugin" explicitly)
+- [ ] .skill/.plugin file is in the Drive folder (or note "skill-only, no .skill" explicitly)
 
 If any artifact failed to land in the folder, resolve it before proceeding.
 
@@ -303,7 +333,7 @@ Adjust if the user specifies otherwise.
 
 Report every artifact with its link:
 - README location: Drive link + Git link (if pushed)
-- .plugin file: Drive link and version (or "skill-only, not applicable")
+- .skill/.plugin file: Drive link and version (or "skill-only, not applicable")
 - Wiki Doc: Google Doc link
 - Registry: link to sheet