Take (or renew) a claim on an item (#1856, #1852).
An implementation claim needs a worktree or a runId so the
one-implementation-per-checkout rule has something to compare, and it is
refused when maxConcurrentImplementations running implementations
already exist or another live implementation uses the same checkout on
this machine. A live claim another session holds is refused unless takeover
is set (the workflow asks first); this session's own claim is renewed.
In Gist mode the checks run again on the Gist's copy when the push
conflicts with another machine: if that copy refuses the claim, nothing is
recorded and the refusal is thrown. The claim is synced: false when it
exists locally only.
Work claims (#1856, #1852): record, finish, release and list in-flight implementations and PR work.