feat: preflight - prerequisites checked and tool paths recorded before wikitool runs; launcher refuses without it (#151, POSIX half)

Files changed:
- .claude/settings.json
- .gitea/workflows/ci.yml
- .gitea/workflows/nightly.yml
- .gitea/workflows/release.yml
- .gitea/workflows/tracker-live.yml
- .github/hooks/wiki-trace.json
- .gitignore
- .vibe/hooks.toml
- AGENTS.md
- CHANGES.md
- EVALS.md
- INSTALL.md
- README.md
- VERSION
- instructions/bootstrap.md
- instructions/preflight.md
- instructions/setup-instance.md
- instructions/upgrade-instance.md
- tools/CONTRACT.md
- tools/README.md
- tools/chemenu/cli.py
- tools/chemenu/commands/dist_cmd.py
- tools/chemenu/commands/docs_verify.py
- tools/chemenu/commands/doctor.py
- tools/chemenu/commands/git_publish.py
- tools/chemenu/commands/migrate_cmd.py
- tools/chemenu/config.py
- tools/chemenu/corpus_cache.py
- tools/chemenu/prerequisites.py
- tools/chemenu/search/ripgrep.py
- tools/chemenu/tests/conftest.py
- tools/chemenu/tests/test_dist_upgrade.py
- tools/chemenu/tests/test_doctor.py
- tools/chemenu/tests/test_preflight.py
- tools/chemenu/toolpaths.py
- tools/preflight.sh
- tools/prerequisites.txt
- tools/run_wikitool.py
- tools/trace-hook
- tools/wikitool

instructions/upgrade-instance.md

@@ -150,8 +150,19 @@ fresh clone ([bootstrap.md](bootstrap.md)), and not for the clone-with-upstream
 
    It writes, and commits nothing.
 
-8. **Republish the skills.** `tools/wikitool instructions sync` - the published skill directories
-   are copies, so until this runs the harness is still offering the previous release's skills.
+8. **Run the preflight, then republish the skills.** The release may need other tools or
+   other libraries than the one it replaced, and `tools/wikitool` refuses to start (exit 42)
+   until the preflight has passed against the new `tools/` - an instance upgrading from a
+   release without one has never run it at all. On its exit 42, show the output verbatim and
+   wait ([preflight.md](preflight.md)):
+
+   ```bash
+   tools/preflight.sh
+   tools/wikitool instructions sync
+   ```
+
+   `instructions sync` is needed because the published skill directories are copies: until it
+   runs, the harness is still offering the previous release's skills.
 
 9. **Verify the machinery, and fix what the release said would need fixing:**