#21 merge conflict output: file names only; reconciler needs hunks/context

Evidence: f0rge merge conflict path parses git apply --check stderr into a filename list, then one-line error "merge conflict: <files>". The reconciling agent must re-derive both sides by hand (git diff base..head, manual 3-way). Fix within cmd/merge.rs conflict path, NO machinery (bitter lesson: no auto-resolve, no merge engine): attach per-file conflict context to the merge record body (both sides"\ hunks / 3-way context) so the resolving agent can act on the record alone. Error line stays one line; richer detail lives in the merge record body (+ optional conflict sidecar in the record). Acceptance: after a conflicted merge, the fail record body contains per-file conflict context and a fresh agent can resolve without re-running git diff. Cross-refs #15/#17/#20 (same merge-flow cluster, coordinate — likely one PR).