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]>

README.md

@@ -12,24 +12,23 @@ A comprehensive Claude Code configuration for professional software development
 - **Security** - Automatic secret detection and blocking
 - **Model Strategy** - Opus for planning, Sonnet for execution (`opusplan`)
 
-## Quick Start
+## Quick Start (lean default)
 
 ```bash
-# 1. Clone this repo
-git clone https://github.com/yourusername/ai-development-scaffold.git
-
-# 2. Backup existing Claude config (if any)
-mv ~/.claude ~/.claude.backup
-
-# 3. Deploy
-cp -r ai-development-scaffold/.claude/* ~/.claude/
-chmod +x ~/.claude/hooks/*.sh
-
-# 4. Verify
-ls ~/.claude/
-# Should show: CLAUDE.md, settings.json, agents/, skills/, hooks/
+git clone https://git.kinec.tech/kinec.tech/ai-development-scaffold.git
+cd ai-development-scaffold
+python3 scripts/lean-install.py global            # dry-run: shows planned changes
+python3 scripts/lean-install.py global --apply    # merge rules into ~/.claude/CLAUDE.md and $CODEX_HOME/AGENTS.md
+python3 scripts/lean-install.py project /path/to/repo --apply   # optional per-project AGENTS.md + CLAUDE.md
+python3 -m unittest discover -s tests             # verify the installer
 ```
 
+Existing files are merged, not replaced (timestamped backup first); settings and credentials are never touched. Details, sources and rejected patterns: [docs/lean-setup.md](docs/lean-setup.md).
+
+### Optional: full library
+
+The sections below describe the full agent/skill/hook library. Install pieces selectively (see [docs/lean-setup.md](docs/lean-setup.md#optional-full-library)); do not move or overwrite `~/.claude`.
+
 ## Documentation
 
 See [.claude/README.md](.claude/README.md) for: