fix(server): bulk worktree launches no longer fail on Git races#17197

fix(server): share fetches for start-from-origin launches

fix: preserve source behavior against current upstream contracts

fix(server): lock worktree branch configuration before checkout

test(server): consolidate concurrent worktree launch coverage

test(server): deduplicate worktree launch fixture import

fix(server): bulk worktree launches no longer fail on Git races