cc @Anthony-Eid

## Why

This is the first step in moving the Git Graph work into the Git UI
crate before continuing with follow-up refactors and feature work. The
goal is for Git UI components and shared Git UI helpers to live in one
crate, so future changes to the Git Graph can reuse existing `git_ui`
code instead of duplicating it.

This PR is not only a filesystem move. While moving `git_graph` into
`git_ui`, a few small dependency and helper boundaries had to change:

- `git_graph` and `git_ui` both needed the same remote parsing and
commit tooltip construction behavior, so those pieces are now shared
from `git_ui`.
- `git_graph` previously depended on `project_panel` to resolve
file-history actions from the project panel selection. After moving
`git_graph` into `git_ui`, keeping that dependency would create an
undesirable `git_ui` -> `project_panel` relationship. The
project-panel-specific action forwarding now lives in `project_panel`,
and calls into exported `git_ui::git_graph` helpers instead.
- `git_graph` initialization now happens through `git_ui::init`, so
downstream crates only need to initialize `git_ui`.

This prepares the codebase for the next planned PRs: splitting the large
`git_graph.rs` implementation into smaller pieces, then adding Git Graph
features such as keeping the main branch lane at index `0`.

## License removal

The removed `crates/git_graph/LICENSE-GPL` file was a symlink to the
repository root `LICENSE-GPL`. The moved code is now inside `git_ui`,
which is also licensed as `GPL-3.0-or-later` and has its own
`LICENSE-GPL` symlink to the same root license file. The code did not
move to a differently licensed crate; it remains covered by the same GPL
license.

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:

- N/A

---------

Co-authored-by: Anthony Eid <anthony@zed.dev>