Isolate worktree checkout admission and bound default workers

Co-authored-by: Neil <4138956+nwparker@users.noreply.github.com>

src/main/git/command-runner/git-exec-file.ts

@@ -75,6 +75,7 @@ async function gitExecFileAsyncUnlocked(
             cwd: options.cwd,
             wslDistro: options.wslDistro,
             tier: options.admissionTier,
+            admissionClass: options.admissionClass,
             signal: options.admissionSignal ?? options.signal
           })
       span?.setAttribute('git.queue_wait_ms', grant.queueWaitMs)

src/main/git/command-runner/git-exec-options.ts

@@ -26,6 +26,8 @@ export type GitExecOptions = {
   observeChildTermination?: (terminated: Promise<void>, writes: boolean) => void
   /** Scheduler priority for this child; status is the safe default. */
   admissionTier?: GitAdmissionTier
+  /** File materialization during worktree creation has a separate concurrency budget. */
+  admissionClass?: 'checkout'
   /** Skips general admission; only for a caller that bounds its own concurrency (worktree deletes). */
   admissionExempt?: true
   /** Native macOS deletion spawns use the shared process worker. */