#3 bug: pr merge on landed PR reports conflict; SEC-08 hash mismatch

f0rge pr merge <uuid> on an already-merged PR: 'merge conflict' instead of 'already landed'. Root: branch payload hashes format-patch stdout; folder payload hashes stored patch.diff (pr create) — different bytes, landed() dedup misses. Also: branch-flow merges never flip canonical pr state to 'merged' (flip only in folder flow). Fix: canonical patch hash + flip canonical pr state on success in both flows.