fix(worktree): run the create-base warm-up on the routed git host (#17794)

The speculative warm-up that runs while the create composer is open resolved
refs and fetched with host Git even when the project's runtime is a WSL distro,
while both the checkout preparation it feeds (`prepareWorktreeCreateForRepo`,
which already resolves `{ wslDistro }` itself) and the real create path run
inside the distro.

The concrete cost was a discarded fetch: `getCanonicalFetchKey` namespaces the
runtime's remote-fetch cache `wsl:<distro>` vs `local`, so the warm-up's fetch
landed in a namespace create never looks at, and create fetched again. On a
Windows host with no usable host-side Git the probes also failed outright, so
that cohort got no warm-up at all.

Thread the project's worktree Git options through the prefetch (resolved by a
non-throwing helper, because an optimistic warm-up must not surface a
repair-required runtime as a failure) so every probe and fetch runs where create
runs. `gitOptions` is a required argument, so a caller cannot drop the routing
silently. Host-routed calls keep their original arity, so macOS, Linux,
native-Windows-host projects, SSH repos and folder workspaces are unchanged.

Narrower than it looks: for a repo under \\wsl.localhost\<distro>\... the probes
were already routed by cwd, and for a repo on a Windows drive letter host Git
and WSL Git read the same on-disk repository, so the answers were already
correct there. What those cohorts gain is a fetch create can reuse; what they
pay is that the probes now run inside the distro (over /mnt/c for drive-letter
repos, which also newly arms the linked-worktree routing probe) and the
speculative fetch now shares create's per-remote fetch queue, as it always has
on native platforms.

Also collapse the three byte-equivalent copies of `hasLocalWorktreeBaseRef`
(create, prefetch, remote-repo create) into one in
git/worktree-base-ref-probe.ts, drop the host-only `hasLocalCommitObject` that
caused the routing bug, and add the first routing assertions on the create-path
consumers of the now-shared probe.

src/main/git/commit-object-ref.ts

@@ -1,5 +1,3 @@
-import { gitExecFileAsync } from './runner'
-
 type GitExec = (args: string[]) => Promise<unknown>
 
 const FULL_GIT_OBJECT_ID_PATTERN = /^[0-9a-f]{40}$/i
@@ -20,7 +18,3 @@ export async function hasCommitObjectViaGitExec(gitExec: GitExec, ref: string):
     return false
   }
 }
-
-export function hasLocalCommitObject(repoPath: string, ref: string): Promise<boolean> {
-  return hasCommitObjectViaGitExec((args) => gitExecFileAsync(args, { cwd: repoPath }), ref)
-}