feat: preflight as a release asset - download, verify, unpack, then run the tree preflight (#151, C)

Files changed:
- .gitea/workflows/release.yml
- CHANGES.md
- INSTALL.md
- README.md
- VERSION
- instructions/dev/testing-conventions.md
- instructions/preflight.md
- tools/CONTRACT.md
- tools/README.md
- tools/chemenu/tests/test_preflight.py
- tools/chemenu/tests/test_preflight_pwsh.py
- tools/preflight.ps1
- tools/preflight.sh

README.md

@@ -39,7 +39,9 @@ Two starting points, depending on what you're doing - full walkthrough in [INSTA
   ```
 
   `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`.
+  instead, its output says what to install - `instructions/preflight.md`. A release carries the
+  same two scripts as assets that download and unpack the stack themselves, for installing
+  without a clone.
 
   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.