docs: update publishing guide with Category and Effect columns

The README table now has Category and Effect columns (added in #1897),
but the publishing guide template still showed the old 3-column format.
Update to match so extension authors know to include both fields.

Made-with: Cursor

Author: imviancagrace

extensions/EXTENSION-PUBLISHING-GUIDE.md

@@ -209,9 +209,22 @@ Edit `extensions/catalog.community.json` and add your extension:
 Add your extension to the Available Extensions table in `extensions/README.md`:
 
 ```markdown
-| Your Extension Name | Brief description of what it does | [repo-name](https://github.com/your-org/spec-kit-your-extension) |
+| Your Extension Name | Brief description of what it does | `category` | Effect | [repo-name](https://github.com/your-org/spec-kit-your-extension) |
 ```
 
+**Category** — pick the one that best fits your extension:
+
+- `docs` — reads, validates, or generates spec artifacts
+- `code` — reviews, validates, or modifies source code
+- `process` — orchestrates workflow across phases
+- `integration` — syncs with external platforms
+- `visibility` — reports on project health or progress
+
+**Effect** — choose one:
+
+- `Read-only` — produces reports without modifying files
+- `Read+Write` — modifies files, creates artifacts, or updates specs
+
 Insert your extension in alphabetical order in the table.
 
 ### 4. Submit Pull Request