Adding agentic marketplace fails when marketplace repo has files with long paths

Describe the bug

So, i have seen a couple of times where someone tries to add an agentic marketplace via "/plugin marketplace add" and fails with obscure error, because the marketplace repo happens to have a file path long enough where, together with the location CLI chooses to clone the repo locally, exceeds the default 260 char max limit in windows.

Example error from CLI when this happens (a couple of specific paths obscured for privacy):

`/plugin marketplace add https://dev.azure.com/microsoft/OS.Developer/_git/NAME_OF_REPO_PROJECT_HERE

Failed to add marketplace: Request plugins.marketplaces.add failed with message: Failed to fetch git marketplace from the provided URL. Please verify that the URL and reference are correct. Details: Command failed: git clone --depth 1 --progress https://dev.azure.com/microsoft/OS.ZZZZZZZZZZZ/_git/NAME_OF_REPO_PROJECT_HERE C:\Users\USER_NAME_HERE\AppData\Local\copilot\marketplaces\https---dev-azure-com-XXXXXXXX-YY-ZZZZZZZZZZ--git-NAME_OF_REPO_PROJECT_HERE

Cloning into 'C:\Users\USER_NAME_HERE\AppData\Local\copilot\marketplaces\https---dev-azure-com-XXXXXXX-YY-ZZZZZZZZZZZ--git-NAME_OF_REPO_PROJECT_HERE'...

remote: Azure Repos

remote: remote: Found 5359 objects to send. (148 ms)

(truncated)

Updating files: 100% (3956/3956), done.

fatal: unable to checkout working tree

warning: Clone succeeded, but checkout failed.

You can inspect what was checked out with 'git status'

and retry with 'git restore --source=HEAD :/'
`

The above error is generally pretty obscure and end user has no idea why adding the marketplace fails.

1. Can we detect this scenario and have better error message to explain to the user what happened? (And ideally help them resolve it by turning off windows path limit or something.)
2. Can we internally clone the repo to a shorter path so that this is less likely to trip? E.g. hash the repo URI or something, instead of having the entire repro URI as part of the path. The existing install path by default is like 100 char long and uses up half the char length already.

Affected version

GitHub Copilot CLI 1.0.81-1

Steps to reproduce the behavior

1. Find a agentic marketplace with some long-ish path inside. (Contact cchui@microsoft.com offline if you need examples.)
2. Run /plugin marketplace add INSERT_MARKETPLACE_URI_HERE
3. Encounter error desscribed above in the bug description

Expected behavior

Ideally succeed, or at least a helpful error so the user understands what went wrong / how to resolve it (e.g. turn off max path limit.)

Additional context

No response

Start with the implementation behind /plugin marketplace add and the git clone path construction, then reproduce the checkout failure on Windows using a marketplace with long paths. Done means the marketplace succeeds with a shorter clone location or reports an actionable explanation of the Windows path-limit failure.