Remove unnecessary intermediate `String` allocations when constructing
`Arc<str>` and `SharedString` values. Use static `SharedString` storage
for fixed worktree picker identifiers.

Validation:

- `cargo fmt --all -- --check`
- `tooling/lints/single-lint owned_string_into_shared -p editor -p
git_ui_core -p toolchain_selector`
- `cargo test -p editor -p git_ui_core -p toolchain_selector`

Release Notes:

- N/A