plugin install fails on repos that use Git LFS: the whole repo's LFS content is downloaded, and one missing object aborts the install #4962

Describe the bug

When a marketplace plugin uses a github source with a path inside a larger repo, copilot plugin install
runs git clone --depth 1 --progress --branch <ref> <url> <cache>/marketplaces/<owner>--<repo> with Git LFS
smudge turned on. Every LFS-tracked file in the repo gets downloaded, including files far outside the
plugin's path. If any one of those objects can't be fetched (for example batch response: Not Found
for an object that was pushed late or never pushed), checkout fails and the install is aborted:

Failed to install plugin: Error: Request plugins.install failed with message: Failed to fetch GitHub plugin <owner>/<repo>: Command failed: git clone --depth 1 --progress --branch main https://github.com/<owner>/<repo> ...\marketplaces\<owner>--<repo>
...
Receiving objects: 100% (29260/29260), 223.35 MiB | 9.17 MiB/s, done.
Downloading <unrelated-data-dir>/.../file.tsv (86 KB)
Error downloading object: ... Smudge error: ... batch response: Not Found
error: external filter 'git-lfs filter-process' failed
fatal: ...: smudge filter lfs failed
warning: Clone succeeded, but checkout failed.

The plugin directory itself contains no LFS files and is about 1.2 MB, but the install downloads about
223 MB of pack data plus every LFS object in the repo, and then fails.

Affected version

Observed on Windows; the clone command is unchanged as of GitHub Copilot CLI 1.0.88.

Steps to reproduce the behavior

1. Register a marketplace plugin with {"source":"github","repo":"<owner>/<repo>","path":"plugins/<name>","ref":"main"}
   where the repo tracks unrelated files with Git LFS (<data-dir>/** filter=lfs).
2. Make one LFS object under <data-dir> unavailable on the server (or just use a large LFS data set).
3. Run copilot plugin install <name>@<marketplace>.
4. The install downloads all LFS content, then fails at checkout with smudge filter lfs failed.

Expected behavior

• The plugin fetch shouldn't run the LFS smudge for content outside the plugin path. At minimum,
  set GIT_LFS_SKIP_SMUDGE=1 for the clone, then git lfs pull --include "<path>/**" only when the
  plugin path actually contains LFS pointers.
• A failure to download content outside the plugin path shouldn't fail the install.
• Ideally, combine this with a partial clone plus sparse checkout of path ([FEATURE] Use sparse checkout for plugin installs — only fetch shipping assets, not entire repo #2399), so only the plugin's files are downloaded.

Additional context

• Workaround: set GIT_LFS_SKIP_SMUDGE=1 before copilot plugin install, and delete the half-cloned
  marketplaces/<owner>--<repo> directory first.
• Related: the cache directory is keyed only by <owner>--<repo>. Two plugins from the same repo on
  different refs look like they share one checkout, so the cache key should probably include the ref.
• OS: Windows 11 (also affects macOS/Linux, because the clone behavior is the same)