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

.gitignore

@@ -141,6 +141,13 @@ npm-debug.log*
 # configured; `doctor` reports which.
 /.wikitool-tasks.json
 
+# Tool paths recorded by the preflight (tools/preflight.sh / .ps1, see
+# instructions/preflight.md): the absolute paths of python, git, rg - and pwsh
+# on Windows - as this machine has them. Per-checkout for the plainest reason of
+# all: a path on one computer means nothing on another. Absent means the
+# preflight has not run, and `tools/wikitool` refuses to start (exit 42).
+/.wikitool-tools.json
+
 # Live-suite tracker profiles (Gitea #156): one file per tracker of the user's own that the
 # live suite may be pointed at (`CHEMENU_LIVE_PROFILE=<name>`,
 # instructions/dev/tracker-testing.md). They carry the same credentials as the file above