feat: bug-report collector tools/bugreport.py and instructions/bug-report.md (#157)

Files changed:
- .gitea/workflows/ci.yml
- CHANGES.md
- INSTALL.md
- VERSION
- instructions/bug-report.md
- instructions/gates.md
- instructions/setup-instance.md
- instructions/upgrade-instance.md
- reports/CONTRACT.md
- tools/README.md
- tools/bugreport.py
- tools/chemenu/tests/test_bugreport.py

instructions/upgrade-instance.md

@@ -247,6 +247,9 @@ fresh clone ([bootstrap.md](bootstrap.md)), and not for the clone-with-upstream
   intermediate state is legitimate and `migrate status` resumes it; what is not legitimate is
   recording a migration with `migrate done` that was not carried out - the version then describes
   a shape the corpus is not in.
+- **A step fails and the cause is not obvious?** Stop rather than improvise, and offer the user a
+  bug report - [bug-report.md](bug-report.md). It runs even when `wikitool` does not start, and
+  only when the user agrees.
 - **`doctor` reports `kb-version` behind `VERSION` after everything is done?** Correct when the
   release's chain was empty or carried only `offered` entries: an offer changes a file the
   instance owns, not the shape of its content, so the content version stays where it was.