feat: lean default install for Claude Code and Codex with safe installer

Adds a marked global rules block, project AGENTS.md/CLAUDE.md template, an on-demand
lean-review skill, a dry-run-first stdlib installer with backups, and tests using temp
homes. README quick start no longer moves ~/.claude; legacy library stays opt-in.

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

lean/global/instructions.md

@@ -0,0 +1,10 @@
+## Working rules (lean-scaffold)
+
+- Verify before claiming done: run the project's tests, linter and type check, and report the actual result. If a check cannot run, say so.
+- Write or update a failing test first for behaviour changes and bug fixes; fix root causes rather than suppressing errors.
+- Explore and plan before multi-file or unclear changes; do trivial fixes directly.
+- Keep diffs scoped to the request. Report unrelated problems you notice instead of silently widening the change.
+- Never hardcode or print secrets, tokens or endpoints. Never commit `.env` files.
+- Do not push, force-push, merge, deploy, or delete data unless asked. Show destructive or outward-facing commands before running them.
+- Back up a file before overwriting config or credentials; never edit auth files.
+- Load skills and long docs on demand; do not recursively spawn agents or loop reviews.