Feat: Integrate with magit worktree feature

[tninja]

With worktree, the multi session are relative nature. The work here is to simplify the worktree creation (basically folder-name = branch-name), it just need to have a worktree-rootname predefined: ai-code-git-worktree-root

[chatgpt-codex-connector]

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[Copilot]

Pull request overview

Adds Magit worktree support to ai-code-interface.el by introducing a centralized worktree root and a transient menu entry to create/visit a new branch worktree (or open worktree status with a prefix arg).

Changes:

• Added ai-code-git-worktree-root customization and new worktree commands in ai-code-git.el.
• Exposed the worktree action via the main transient menu (w) in ai-code.el.
• Added ERT tests covering the new worktree functions in test/test_ai-code-git.el.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
ai-code-git.el	Adds centralized worktree root defcustom plus commands to create a worktree branch and dispatch worktree actions.
ai-code.el	Wires the new worktree action into the transient menu (w).
test/test_ai-code-git.el	Introduces unit tests for the new worktree branch/action commands.

[tninja]

This is initial attempt on worktree feature. I'll merge this PR and use it firstly. Will submit separate PR to improve / fix this feature later after I get more knowledge about worktree.

[tninja]

addressing #215