fix: eliminate dead code + add stale worktree pre-cleanup in _merge_reweave_pr

- Combined superset assertion and merge computation into single loop
  (removed duplicate scalar-to-list normalization)
- Added worktree remove --force before worktree add to handle prior
  crash leaving stale worktree (SIGKILL, OOM, power loss)

lib/merge.py

@@ -544,7 +544,8 @@ async def _merge_reweave_pr(branch: str) -> tuple[bool, str]:
     if not changed_files:
         return False, "no .md files changed"
 
-    # Create worktree from origin/main
+    # Pre-cleanup: remove stale worktree/branch from prior crash (SIGKILL, OOM, etc.)
+    await _git("worktree", "remove", "--force", worktree_path)
     await _git("branch", "-D", clean_branch)
     rc, out = await _git("worktree", "add", "-b", clean_branch, worktree_path, "origin/main")
     if rc != 0:
@@ -582,29 +583,9 @@ async def _merge_reweave_pr(branch: str) -> tuple[bool, str]:
                 # to cherry-pick. (Theseus: loud failure, not silent retry)
                 return False, f"frontmatter parse failed on {fpath} — manual review needed"
 
-            # Superset assertion: branch's edge set must be a superset of main's.
+            # Superset assertion + merge in one pass.
             # Reweave only adds edges. If branch is missing an edge that main has,
             # the branch was based on stale main — union is safe (adds both).
-            for field in REWEAVE_EDGE_FIELDS:
-                main_list = main_fm.get(field, [])
-                branch_list = branch_fm.get(field, [])
-                if not isinstance(main_list, list):
-                    main_list = [main_list] if main_list else []
-                if not isinstance(branch_list, list):
-                    branch_list = [branch_list] if branch_list else []
-
-                main_keys = {str(v).strip().lower() for v in main_list if v}
-                branch_keys = {str(v).strip().lower() for v in branch_list if v}
-
-                missing = main_keys - branch_keys
-                if missing:
-                    logger.warning(
-                        "Reweave merge: %s field '%s' — branch missing edges from main: %s",
-                        fpath, field, missing,
-                    )
-                    skipped_non_superset.append(f"{fpath}:{field}")
-
-            # Collect merged edge fields for string-level splicing
             merged_edges = {}
             for field in REWEAVE_EDGE_FIELDS:
                 main_list = main_fm.get(field, [])
@@ -613,6 +594,19 @@ async def _merge_reweave_pr(branch: str) -> tuple[bool, str]:
                     main_list = [main_list] if main_list else []
                 if not isinstance(branch_list, list):
                     branch_list = [branch_list] if branch_list else []
+
+                # Superset check
+                main_keys = {str(v).strip().lower() for v in main_list if v}
+                branch_keys = {str(v).strip().lower() for v in branch_list if v}
+                missing = main_keys - branch_keys
+                if missing:
+                    logger.warning(
+                        "Reweave merge: %s field '%s' — branch missing edges from main: %s",
+                        fpath, field, missing,
+                    )
+                    skipped_non_superset.append(f"{fpath}:{field}")
+
+                # Collect merged edges for string-level splicing
                 if main_list or branch_list:
                     merged_edges[field] = _union_edge_lists(main_list, branch_list)