Issue #1789: chore(repo): set up lfs for vscode extensions vendored into the repo (#2516)

For now, only move existing extensions already on main into lfs. Will not rewrite history to remove the previously added versions.

This means that repository size is not decreased by this commit. But it should not grow so much any more every time code-server is updated.

```
❯ brew install git-lfs
❯ git lfs track "*.vsix"
Tracking "*.vsix"
❯ cat .gitattributes
ci/secrets/** filter=git-crypt diff=git-crypt
*.vsix filter=lfs diff=lfs merge=lfs -text
❯ git add .gitattributes
```

Issue #1789: chore(repo): update docs with git-lfs instructions for codeserver in Extensions.md

Author: jiridanek

.gitattributes

@@ -1 +1,2 @@
 ci/secrets/** filter=git-crypt diff=git-crypt
+*.vsix filter=lfs diff=lfs merge=lfs -text

README.md

@@ -18,6 +18,7 @@ Make sure the following tools are installed in your environment:
  - pipenv
  - make (on macOS install/use: gmake)
  - curl
+ - git, git-lfs (for `*.vsix` files)
 
 ### Installation
 Clone this repository to your local machine:

codeserver/Extensions.md

@@ -18,6 +18,10 @@ List of extensions used:
   - Jupyter Cell Tags [ms-toolsai.vscode-jupyter-cell-tags]
   - Jupyter Slide Show [ms-toolsai.vscode-jupyter-slideshow]
 
+## Prerequisites
+
+- git-lfs (required to handle `.vsix` extension files stored in this repository)
+
 ## Update process
 
 To update the extensions, we suggest running a code-server image locally using Podman and manually checking the extensions tab, in a way that you can identify the version available to the version of code-server that you are running on, since some extensions are not available to some code-server versions.
@@ -27,3 +31,15 @@ To update the extensions, we suggest running a code-server image locally using P
 3. Search for the desired version of the extension that you want to install
 4. Under `Resources`, you can click on `Marketplace` to see more details on `Open VSX Registry`
   1. On `Open VSX Registry` you can select a specific version or click on `Download` to download the `vsix` file
+
+## Git LFS integration
+
+Extension packages (`.vsix` files) vendored into this repository are automatically managed by Git LFS via `.gitattributes` rules (`*.vsix filter=lfs diff=lfs merge=lfs -text`). You don't need to manually run `git lfs track` for new `.vsix` files.
+
+When adding or updating extensions, place the downloaded `.vsix` artifacts under the appropriate `codeserver/ubiX-python-Y/utils/` directory. Commit them normally; they will be stored as LFS objects.
+
+## Troubleshooting
+
+- If you see errors like "End of central directory record signature not found" when installing `.vsix` files, it usually means LFS pointers were checked out instead of the actual binaries.
+  - For local development: run `git lfs install` once, then `git lfs pull` to fetch LFS content.
+  - For CI or other environments: ensure the Git checkout step enables LFS, or run `git lfs pull` after checkout.