#17 merge branch: fork-drifted diff can silently delete shared .forge/prs records

Evidence: dog-issue forked before dog-pr merged; its main..head diff contained deletions of already-landed PR records (t1/t2/78e0c97e folders); orchestrator avoided data loss by merging the folder payload (disjoint range) instead of the branch diff. Acceptance: merging a branch whose base predates other landings must NOT silently delete forge state; deletions of .forge/prs/* surface as conflicts (file list + stop), never applied silently. File: cmd/merge.rs (base-aware diff / deletion surfacing). Bitter-lesson: no merge engine, no auto-resolution — surface the conflict, list files, stop; the reconciling agent decides.