Improve error message when running git-sweep in non-repository directory

[Copilot]

When git-sweep is run in a directory that is not a git repository, it previously showed a technical error message that was not user-friendly:

error: git rev-parse --is-inside-work-tree: exit code 128: exit status 128

This change improves the error handling to detect when the underlying git command fails because we're not in a git repository, and returns the same clear message that Git itself provides:

error: fatal: not a git repository (or any of the parent directories): .git

The fix is minimal and surgical - it only modifies the IsInsideWorkTree function in internal/git/helpers.go to detect the "not a git repository" error in stderr and return an appropriate user-friendly message. All existing functionality remains unchanged, and the tool continues to work normally when run in actual git repositories.

Before:

$ cd /tmp
$ git-sweep
error: git rev-parse --is-inside-work-tree: exit code 128: exit status 128

After:

$ cd /tmp  
$ git-sweep
error: fatal: not a git repository (or any of the parent directories): .git

Fixes #7.

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