perf(worktrees): fetch the base while the host's create picks a name (#26007)

* perf(worktrees): look up a branch's PR only after a name collision on the host's create

The host's create (agent.launch, worktree.create, the phone and the CLI) called gh pr list for every
create, a 1-3 s network round trip, while the desktop create probes only after a branch collision. A
branch with an open PR exists on the remote, so the branch-conflict check already moves past it.

* test(worktrees): pin first-name PR lookup skip on the host's create

The runtime service specs still expected a PR lookup for a free first
name. Update them to the desktop-parity behavior: no lookup without a
branch collision, and a free push-target name is kept. Pin the exact
suffixed name, and that a later name with a PR is skipped.

* perf(worktrees): overlap the host create's base fetch with naming

Keep the first-name PR lookup (restores main's naming) and take it off
the critical path instead: start the base-ref refresh before naming and
await it after, as the desktop create does, so the lookup and the fetch
run concurrently. A refresh rejection while naming fails stays handled.

* test(worktrees): pin the overlapped origin fetch on the host's create

Cover the best-effort fetch origin path: it runs while the first-name PR
lookup is pending, and its rejection while naming fails stays handled.

src/main/runtime/runtime-worktree-create-git.test.ts

@@ -0,0 +1,32 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import type * as GitHubClient from '../github/client'
+
+const getPRForBranchMock = vi.hoisted(() => vi.fn())
+
+vi.mock('../github/client', async (importOriginal) => ({
+  ...(await importOriginal<typeof GitHubClient>()),
+  getPRForBranch: getPRForBranchMock
+}))
+
+import { getLocalGitHubPrForBranch } from './runtime-worktree-create-git'
+
+beforeEach(() => {
+  getPRForBranchMock.mockReset()
+  getPRForBranchMock.mockResolvedValue(null)
+})
+
+describe('getLocalGitHubPrForBranch', () => {
+  it('routes the PR lookup through the WSL distro', async () => {
+    await getLocalGitHubPrForBranch('/repo', 'app', { wslDistro: 'Ubuntu' })
+
+    expect(getPRForBranchMock).toHaveBeenCalledWith('/repo', 'app', null, null, null, {
+      localGitExecOptions: { wslDistro: 'Ubuntu' }
+    })
+  })
+
+  it('uses the plain lookup without project git options', async () => {
+    await getLocalGitHubPrForBranch('/repo', 'app', {})
+
+    expect(getPRForBranchMock).toHaveBeenCalledWith('/repo', 'app')
+  })
+})