5.1.0

release: v5.1.0 — close the loop (landing as a phase)

The run lifecycle now spans launch -> settle -> preview -> (resolve) -> landed
on origin, without leaving coxpit:
- no-op/blocked visibility + sibling overlap + land order
- base-drift warning + conflict preview (vs the origin target, incl. uncommitted
  worktree changes)
- origin-aware land: rebase/squash onto origin/<target>, push as
  coxpit/<task-slug>-r<id>, PR against the target
- in-app conflict resolution: coxpit drives git, the run's own agent edits the
  markers, then it lands automatically

Full-loop live-validated (real gh + real agent): clean land and agent-resolved
conflict both opened clean PRs on a throwaway repo.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>