Office: overlap guard should also see workers hired without the queue that have no PR yet #3427

Follow-up from #3119 / #3426. The server-side hire guard (apps/agent-office/live/overlap.ts) compares a new task's files against open PRs and queued tasks. A worker hired directly (office-workers hire) that hasn't opened a PR yet isn't seen until it does, so two hires minutes apart on the same files still both start. Option: guess files from each running worker's first prompt (the office keeps it) or its worktree's git diff --name-only against main, and add those to clashOf. Size S.