feat: changelog entries stay compact - one paragraph per topic, a size budget in docs verify and version release, version bump names required migration documents (#184)

Files changed:
- CHANGES.md
- DEVELOPMENT.md
- VERSION
- instructions/dev/stack-build/SKILL.md
- instructions/dev/version-parts.md
- instructions/migrate-corpus.md
- tools/CONTRACT.md
- tools/chemenu/commands/docs_verify.py
- tools/chemenu/commands/version_cmd.py
- tools/chemenu/tests/test_docs_verify.py
- tools/chemenu/tests/test_version_cmd.py
- tools/chemenu/version.py

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

instructions/dev/stack-build/SKILL.md

@@ -59,8 +59,10 @@ session, or a fresh one after `/clear` - assume the second, and work from the bo
    model.
 
    Never edit `VERSION` or the entry's heading by hand - `bump` writes both, and `docs verify`
-   fails a tree where they disagree. Then write the entry's body - `bump` deliberately leaves it
-   empty, the same way `new` leaves the prose. A `--major` bump needs `--breaking` and either a
+   fails a tree where they disagree. Then decide the entry's prose: for a smaller change its bump
+   title is the whole entry; a larger one gets a one-paragraph `###` section for its topic, or
+   rewrites the one its topic already has. The form and the budget `docs verify` holds it to are
+   `instructions/dev/version-parts.md` § The candidate model. A `--major` bump needs `--breaking` and either a
    migration document or `--no-migration`; `instructions/dev/version-parts.md` has all of it.
 
    Prose-only changes (`README.md`, `INSTALL.md`, `EVALS.md`) and the workflows under `.gitea/`