The command palette action `git graph: copy commit tag` still deploys a
context picker.

<img width="276" height="169" alt="SCR-20260515-mttj"
src="https://github.com/user-attachments/assets/635e93eb-8617-4a99-8727-ca32a587d8e2"
/>

But the context menu now uses a submenu when there are multiple tags:

0 tags:

<img width="383" height="284" alt="SCR-20260515-mtgo"
src="https://github.com/user-attachments/assets/b9520725-887e-42ff-910d-08264b2976ba"
/>

1: tag:

<img width="524" height="298" alt="SCR-20260515-mtmb"
src="https://github.com/user-attachments/assets/17eaf2fa-81f2-43d2-9175-ba99be8faa23"
/>

multiple tags:

<img width="471" height="288" alt="SCR-20260515-mtau"
src="https://github.com/user-attachments/assets/881705e3-9281-4be4-9d44-931b4810352a"
/>

Self-Review Checklist:

- [X] I've reviewed my own diff for quality, security, and reliability
- [X] Unsafe blocks (if any) have justifying comments
- [X] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [X] Tests cover the new/changed behavior
- [X] Performance impact has been considered and is acceptable

Release Notes:

- Improved `Copy Tag` workflow from Git Graph context menu to deploy a
submenu instead of a modal picker.