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

README.md

@@ -33,10 +33,13 @@ Two starting points, depending on what you're doing - full walkthrough in [INSTA
   committed**. Publish them once:
 
   ```bash
-  cd tools && python3 -m venv .venv && .venv/bin/pip install -r requirements.txt && cd ..
+  tools/preflight.sh            # checks python/git/rg, records their paths, creates tools/.venv
   tools/wikitool instructions sync
   ```
 
+  `tools/wikitool` refuses to start (exit 42) until the preflight has passed; if it stops
+  instead, its output says what to install - `instructions/preflight.md`.
+
   That copies each `instructions/<name>/SKILL.md` into `.agents/skills/` (GitHub Copilot, Codex
   CLI, Mistral Vibe) and `.claude/skills/` (Claude Code). Re-run it after changing a skill.
   Full procedure: `instructions/bootstrap.md`.