Reweave PRs modify existing files (appending YAML edges). Cherry-pick
fails ~75% when main moves between PR creation and merge.
_merge_reweave_pr() reads each changed file from both main HEAD and
branch HEAD, unions the edge arrays (order-preserving, main-first),
and writes the result. Eliminates merge conflicts structurally.
Key design decisions (Ganymede + Theseus approved):
- Order-preserving dedup: main's edges first, branch-new appended
- Superset assertion: logs warning if branch missing main edges
- Uses main's body text (reweave only touches frontmatter)
- Loud failure on parse errors (no cherry-pick fallback)
- Append-only contract: reweave adds edges, never removes
18 tests covering parse, union, serialize, superset, and full workflow.
