fix: comparison and source pages accept the sources: cite add writes; sources may cite sources (#173)

Files changed:
- CHANGES.md
- VERSION
- instructions/wiki-ingest/SKILL.md
- kb/CONTRACT.md
- tools/CONTRACT.md
- tools/chemenu/commands/cite_cmd.py
- tools/chemenu/commands/xref.py
- tools/chemenu/tests/test_cite_cmd.py
- tools/chemenu/tests/test_page_ops.py
- tools/chemenu/tests/test_type_resolver.py
- tools/chemenu/tests/test_xref.py
- types/comparison.md
- types/comparison.schema.yaml
- types/source.md
- types/source.schema.yaml

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SnAJ7Z3CpVD3PRbN73QtU2

types/source.md

@@ -6,7 +6,7 @@ schema: types/source.schema.yaml
 subtype_field: source_type
 base_dir: sources
 title_prefix: "Source - "
-page_ref_fields: [entities, concepts]
+page_ref_fields: [entities, concepts, sources]
 capture_fields: [fidelity, authority]
 guidance: types/source.guidance.md
 layout:
@@ -42,6 +42,7 @@ how to write a conforming page is [types/source.guidance.md](source.guidance.md)
 | `tags` | No | Navigation tags for categorization |
 | `entities` | No | Titles of the entities mentioned in this source |
 | `concepts` | No | Titles of the concepts mentioned in this source |
+| `sources` | No | Titles of *other* source pages this source cites, written by `wikitool cite add` - never its own title |
 | `summary` | Yes | One-liner for `kb/index.md` |
 
 ## Template