fix: config creation when remote is empty

[sammcj]

The scenario this PR fixes:

When trying to create a new session (for the first time) Claude Squad errors with:

$ cd ~/git/sammcj/mcp-devtools
$ cs

# (Press 'n' to start a new session, entered 'devtools' as the name)

failed to setup git worktree: failed to cleanup existing branch: failed to update repository config after removing branch samm/devtools: remote config: empty URL

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[sammcj]

I have read the CLA Document and I hereby sign the CLA