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/migrate-corpus.md

@@ -114,7 +114,8 @@ declared by a migration document under `instructions/migrations/`. A single page
 A migration that a distributed instance must also run is a `manual: true` instruction under
 `instructions/migrations/<version>-<slug>.md`, carrying `migrates_to:` and `migration_kind:`.
 `tools/wikitool migrate status` builds the outstanding chain from those files, and `version
-bump` refuses a compatibility-breaking release that has none.
+bump` refuses a compatibility-breaking release that has none - and names a required one by path
+in that release's changelog entry, which is where an operator reading the release notes finds it.
 
 Write it for a reader who has the new machinery and the old content, and who is not you: what
 changed, which pages are affected, how to tell a migrated page from an unmigrated one, and what