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/skills/lean-review/SKILL.md

@@ -0,0 +1,18 @@
+---
+name: lean-review
+description: One consolidated, evidence-based review of the current diff (correctness, failure handling, tests, security only when warranted). Use on demand before finishing or opening a PR.
+disable-model-invocation: true
+allowed-tools: Read, Grep, Glob, Bash(git diff:*), Bash(git status:*), Bash(git log:*)
+---
+
+# Lean diff review
+
+Read-only. Review `git diff` against the base branch (or the target the user names) once, in a single pass. Do not spawn other agents.
+
+1. Run the project's tests/lint/type check if not already run; note results.
+2. For each suspicion, it is only a candidate. Try to refute it: read callers, look for existing guards, confirm the path is reachable.
+3. Report only candidates you could not refute, each with: file:line, a concrete failing input or state, and the wrong result. Drop refuted ones; list unresolved ones separately with what you checked.
+4. Check: wrong value captured or boundary arithmetic; failures swallowed or missing timeouts on external calls; tests that pass without exercising the change; new behaviour without a test.
+5. Add a security section only if the diff touches auth, secrets, input parsing, or dependencies.
+
+Output at most 10 findings, worst first, then a one-line verdict. No style nits.