new: scaffold materializes a schema default only for a required field

Files changed:
- CHANGES.md
- VERSION
- instructions/CONTRACT.md
- tools/CONTRACT.md
- tools/chemenu/commands/new_page.py
- tools/chemenu/tests/test_new_page.py
- types/type-spec.md

tools/chemenu/commands/new_page.py

@@ -11,6 +11,10 @@ deterministic and stored in /types/; the content is judgment and provided by the
 
 Frontmatter defaults, enum validity, and required-ness all come from the
 type's `.schema.yaml` (via `TypeResolver`) - nothing here re-declares them.
+A schema `default:` is materialized only for a field the schema also lists
+in `required:` - an optional field's default is a reader-side assumption
+(what a missing field means), and writing it into every scaffolded page
+would turn that assumption into a stated claim instead (Gitea #109).
 Directory placement for subtype-driven types (currently just entities) also
 comes from the type-spec, via its `layout:` frontmatter (see
 `TypeResolver.get_layout`) - not a hand-maintained Python dict.
@@ -74,12 +78,22 @@ def _build_frontmatter(
     `explicit` supplies every CLI-derived value the caller already has;
     fields not in `explicit` get a type-appropriate default (today's date for
     date-formatted fields, the scaffold placeholder for `summary`, the
-    schema's own `default:` where declared, an empty list for arrays), or are
-    omitted entirely if optional with no sensible default (e.g.
-    `source_url`). This is what lets frontmatter shape - and scaffold-time
-    defaults like `provenance: general` - follow the schema instead of being
-    hand-declared per CLI command.
+    schema's own `default:` where declared *and the field is required*, an
+    empty list for arrays), or are omitted entirely if optional with no
+    sensible default (e.g. `source_url`). This is what lets frontmatter
+    shape - and scaffold-time defaults like `provenance: general` - follow
+    the schema instead of being hand-declared per CLI command.
+
+    A `default:` on an *optional* field (e.g. `instruction.obligation`) is
+    deliberately not materialized here: it documents what a reader should
+    assume when the field is absent, not what the scaffold should write.
+    Writing it anyway turned every scaffolded instruction into one that
+    falsely claims `obligation: required` - a migration-only field - and
+    the same read/write distinction is what the schema's own `default:`
+    doc-comment (`types/instruction.schema.yaml`) already draws (Gitea
+    #109).
     """
+    required = set((schema or {}).get("required") or [])
     frontmatter: Dict[str, Any] = {"type": type_path}
     for field_name, field_schema in (schema or {}).get("properties", {}).items():
         if field_name == "type":
@@ -105,7 +119,7 @@ def _build_frontmatter(
             frontmatter[field_name] = resolved_author
         elif field_schema.get("format") == "date":
             frontmatter[field_name] = today
-        elif "default" in field_schema:
+        elif "default" in field_schema and field_name in required:
             frontmatter[field_name] = field_schema["default"]
         elif field_schema.get("type") == "array":
             frontmatter[field_name] = []