This PR adds a "Resolve with Agent" button in each merge conflict block,
as well as "Resolve Conflicts with Agents" button on a notification for
resolving conflicts across all the files that have any. When clicking on
either of these buttons, the agent panel opens up with a template prompt
auto-submitted. For the first case, the specific content of the merge
block is already attached as context for the agent to act quickly, given
it's a local and small context. For the second case (all conflicts
across the codebase), the prompt just indicates to the agent which files
have conflicts and then it's up for the agent to see them. This felt
like a simpler way to go as opposed to extracting the content for all
merge conflicts across all damaged files.

Here's how the UI looks like:

<img width="550" height="1964" alt="Screenshot 2026-02-21 at 11  04@2x"
src="https://github.com/user-attachments/assets/96815545-ba03-40e5-9cb0-db0ce9588915"
/>

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Before you mark this PR as ready for review, make sure that you have:
- [x] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects
- [x] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- Git: Added the ability to quickly resolve merge conflicts with the
agent.

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>