chore: add step to clean up failed clone directory

[iv1310]

Add submodule cloning support via native Go implementation

Summary

This PR fixes an issue where a failed clone (e.g. due to authentication errors) leaves behind a partial .git directory, causing subsequent attempts to incorrectly detect the repo as already cloned and skip the clone step entirely.

Changes

• Added a repoHasContent helper that validates an existing repo by checking for a resolvable HEAD, a valid commit object, and at least one file in the working tree beyond .git.
• When validation fails, the stale directory is removed and the clone proceeds fresh.
• Added cleanup logic after a failed git.CloneContext call to remove any partially created repo directory, preventing dirty state from persisting across retries.

Behavior

• Before: A failed clone left a partial .git directory. On the next run, the tool detected this as an existing repo and skipped cloning, leading to missing workspace files and downstream errors (e.g. devcontainer.json: no such file or directory).
• After: A failed clone cleans up after itself. If a stale or incomplete repo directory is detected on the next run, it is removed and a fresh clone is attempted.

envbuilder vdev+602df39 - Build development environments from repositories in a container
#1: 📦 Cloning https://github.com/dang-org/dung#development to /workspaces/dung...
#1: 🔒 Using HTTP basic authentication!
Failed to clone repository: clone "https://github.com/dang-org/dung#development": authentication required: Invalid username or token. Password authentication is not supported for Git operations.
Falling back to the default image...
No Dockerfile specified, looking for a devcontainer.json...
Building in Devcontainer mode using /.devcontainer/coder/devcontainer.json
error: open devcontainer.json: open /workspaces/dung/.devcontainer/coder/devcontainer.json: no such file or directory
envbuilder vdev+602df39 - Build development environments from repositories in a container
#1: 📦 Cloning https://github.com/dang-org/dung#development to /workspaces/dung...
#1: 🔒 Using HTTP basic authentication!
#1: 📦 The repository already exists! [170.774µs]