fix: improve branch resolution in CreateCommit tool

What does this merge request do and why?

This MR updates the CreateCommit tool to create branch and MR based on the user input:

• If no branch is specified, the tool automatically creates a new branch (duo-edit-<timestamp>) based on the project’s default branch or the start_branch specified by the user.
• After a successful commit, merge request is automatically created from the resulting branch (unless explicitly instructed not to).
• If a branch is explicitly provided, the commit is created directly on that branch.

How to set up and validate locally

1. Run Duo Agent platform and GDK.
2. Ask Duo to update some file and create commit with the change.
3. Ensure commit was successfully created together with MR.

Screen Recording 2025-10-15 at 14.04.41.mov

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.

Related to gitlab-org/gitlab#574714