arena: cleanup misses Grok copies; architect checkpoint sketch placement #214

Open findings from PR #201 round 3 (third FAIL, stopped for the operator).

1. Grok Build's native isolation makes standalone repo copies, not git worktrees (recorded grok arena-run relay-go6dzbwe: 5 copies under grok-home/worktrees/w-relay/subagent-*, each with its own .git, alive at run end). git worktree remove --force exits 128 on them and step 4 gives no other removal, so PORTING's arena note ("the next run finds none of its candidate files … worktrees") is false on Grok. Fix: step 4 deletes a native copy that is not a git worktree, or PORTING names the case.
2. arena Outputs "code the calling task ships goes onto the branch it works on" plus architect's "the synthesis can ship as its own commit either way" lets a lead commit an /architect-with-checkpoint sketch before sign-off, which the checkpoint oracle FAILs (shape of results/2026-10-04-pr136.md:38). Say a sketch awaiting sign-off stays outside the project.
3. Oracle-side (see evals: oracle follow-ups from the #202 review #207 item 5 and evals: oracles should not FAIL or PASS on a killed run's missing evidence #208): arena-candidates-own-worktrees counts worktrees after cleanup; classify misses $out/... after cd "$out" and /private/var/folders/ paths.