checkpoint resume: harden auto-detection and clash message

Auto-detection now only treats local branches as branch targets, so revision syntax like HEAD and remote-only branch names fall through to commit resolution instead of misrouting via branchCommit's origin/ fallback; the local-branch check also runs before the remote checkpoint fetch, sparing hex-named branches a network round-trip. The worktree- clash message points at 'entire checkpoint resume <id>' in the owning worktree instead of the session picker, cancelling the picker prints feedback, and the ambiguous-prefix error names checkpoints.