fix(server): actionable error when creating a worktree in a commit-less repo

[sideeffffect]

What Changed

GitVcsDriver.createWorktree now verifies that the base ref actually points at a commit before running git worktree add. When it doesn't (an unborn HEAD), it fails with a clear, actionable GitCommandError:

Cannot create a worktree from '<ref>' because it does not point at any commit yet. Make an initial commit in this repository, or start the thread in the current checkout instead.

Added an integration test covering a freshly-git init'd repo with no commits.

Why

Starting a thread in New worktree mode inside a repository that has no commits yet (e.g. a freshly git init'd directory, or a project nested under a barely-initialized parent repo) failed to start the agent. git worktree add -b <tmp> <path> <base> cannot branch from an unborn HEAD, so git aborts with a cryptic fatal: invalid reference: <ref> that surfaced to the user with no guidance on how to proceed.

A worktree fundamentally cannot be created from a repo with no commits, so this detects the condition up front and explains what to do instead (make an initial commit, or use the current checkout). Scope is limited to the pre-flight check plus its error message; healthy repos are unaffected — the extra git rev-parse --verify --quiet <ref>^{commit} resolves and creation proceeds exactly as before.

Note: an alternative would be to silently fall back to the current checkout in this case. I went with an explicit, actionable error to avoid changing the meaning of an explicit "New worktree" selection — happy to switch to a fallback if you'd prefer.

UI Changes

None.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes (N/A)
• I included a video for animation/interaction changes (N/A)

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Note

Low Risk
Small, localized guard on worktree creation with clearer errors; normal repos only pay one extra git call on the existing path.

Overview
New worktree creation in repos with no commits (unborn HEAD) used to fail with git’s opaque fatal: invalid reference when starting a thread. GitVcsDriverCore.createWorktree now runs rev-parse --verify --quiet <ref>^{commit} before git worktree add; if the ref resolves to nothing, it returns a GitCommandError telling the user to make an initial commit or use the current checkout instead of running worktree add.

Repos that already have commits behave the same after the extra rev-parse. An integration test covers init with no commits: expects the new error text, and confirms no worktree directory is left behind.

^{Reviewed by Cursor Bugbot for commit 9c53228. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Return actionable error from createWorktree when base ref has no commit

When creating a worktree from a branch with an unborn HEAD (e.g. a repo with no commits), GitVcsDriverCore.createWorktree now runs rev-parse --verify --quiet <ref>^{commit} before invoking git worktree add. If the ref resolves to nothing, it returns a GitCommandError with a message explaining the ref has no commit, rather than letting the git command fail with an opaque error.

^{Macroscope summarized 9c53228.}

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Approvability

Verdict: Approved

Straightforward UX improvement that adds early validation when creating a worktree in a commit-less repository, returning an actionable error message instead of a cryptic git failure. Normal-path behavior is unchanged, scope is limited, and test coverage is included.

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