fix: libgit2 branch checkout to avoid half-switched HEAD

[ishanray]

Problem

When checkout_branch encountered an error (e.g., dirty working tree conflicts or missing target branch), the repository could be left in an inconsistent state with HEAD pointing to a branch that was never actually checked out.

This happened because the previous implementation called set_head() before checkout_head(). If the checkout failed, HEAD was already updated, leaving the repo in a half-switched state.

Solution

Reorder the libgit2 operations:

1. Resolve the target ref first — validate the branch exists via revparse_single()
2. Checkout the tree — use checkout_tree() to switch the working directory
3. Update HEAD last — only call set_head() after a successful checkout

This ensures HEAD is never updated unless the checkout succeeds, making the operation atomic from the user's perspective.

How to Reproduce (Before Fix)

1. Open a workspace in CodexMonitor
2. Have uncommitted changes that would conflict with a target branch
3. Use the branch switcher (Cmd+B) to switch branches
4. Observe that if the checkout fails due to conflicts, git status may show you on the new branch but with the old tree, or vice versa

Alternatively:

1. Try to switch to a branch name that doesn't exist
2. Before this fix, HEAD could be set to an invalid ref before the error was raised

Testing

Added a unit test checkout_branch_missing_does_not_change_head that:

• Creates a temp repo with an initial commit
• Attempts to checkout a non-existent branch
• Asserts HEAD remains unchanged after the error

[Dimillian]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep them coming!

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