0.6.0: claims across computers

• Several computers on one project (work.remote, references/machines.md): each claim first creates the ref grind-claim/<key> on the repository host. Creating an existing ref fails, so one computer wins; renewals and take-overs of expired claims are fast-forward updates, so they have one winner too. The ref's empty commit names machine, session and expiry. next defers items another computer holds; checkpoint renews; done, drop and session release delete the ref; sync marks held items and gives back stale ones of its own; grw.py remote lists every claim. On issues, a label (agent-working) and one marked comment. ticket_first refuses claims on items with no issue or PR, so discovery on both computers goes through the duplicate-checked tracker. Hosts that cannot be reached refuse the claim unless "offline": "local". Four tests with a fake host; a live test on GitHub (four simultaneous creates, one winner). Numbered 0.6.0 because 0.5.0 is in progress on another branch.