docs: add CLAUDE.md for project guidance

- Introduced CLAUDE.md to provide comprehensive guidance on the macOS Emacs building system, including project overview, common commands, architecture details, and testing instructions.

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with
+code in this repository.
+
+## Project Overview
+
+macOS Emacs building system with Ruby and Go components:
+
+- **Ruby script** (`build-emacs-for-macos`): Main build script for creating
+  self-contained Emacs.app bundles
+- **Go CLI tool** (`emacs-builder`): Packaging, signing, notarization, and
+  release management
+- **Dual dependency management**: Nix (preferred) or Homebrew
+
+## Common Commands
+
+```bash
+# Environment setup (Nix preferred)
+nix develop                       # Default macOS 11 SDK
+nix develop .#macos{11-15,26}     # Target specific SDK version
+
+# Go development
+make build                        # Build emacs-builder CLI
+make test                         # Run tests with race detection
+make lint                         # Run golangci-lint
+make format                       # Format with gofumpt
+
+# Ruby development
+bundle exec rubocop               # Lint (with development group)
+
+# Build Emacs
+./build-emacs-for-macos           # Build from master
+./build-emacs-for-macos emacs-29.4
+```
+
+## Architecture
+
+### Ruby Build Script (`build-emacs-for-macos`)
+
+Single-file Ruby script (~2500 lines) that:
+
+- Downloads source tarballs from emacs-mirror/emacs on GitHub
+- Configures and compiles Emacs with native-comp, tree-sitter support
+- Creates self-contained .app bundles by embedding/relinking dependencies
+- Uses `ruby-macho` gem for Mach-O binary manipulation (RPATH handling)
+
+### Go CLI (`cmd/emacs-builder/`)
+
+Uses `urfave/cli/v2` framework. Key packages in `pkg/`:
+
+- `cli/`: Commands (plan, sign, sign-files, notarize, package, release, cask)
+- `sign/`: macOS code signing via `codesign`
+- `notarize/`: Apple notarization workflow via `notarytool`
+- `release/`: GitHub release management
+- `dmgbuild/`: DMG creation using Python dmgbuild
+- `plan/`: Build plan JSON parsing and management
+
+### Nix Environment (`flake.nix`)
+
+- Multi-SDK support: macOS 11-15, 26 via `.#macos{11,12,13,14,15,26}`
+- Excludes ncurses intentionally (links against system version for TUI)
+- Sets `MACOSX_DEPLOYMENT_TARGET`, `DEVELOPER_DIR`, `NIX_LIBGCCJIT_*`
+
+## Testing
+
+```bash
+make test                         # All Go tests
+go test ./pkg/release/...         # Single package
+go test -run TestName ./pkg/...   # Single test
+```
+
+Tests use `_test.go` suffix alongside source files.
+
+## Working Directories
+
+- `sources/`: Downloaded/extracted Emacs source (gitignored)
+- `builds/`: Build outputs and .app bundles (gitignored)
+- `patches/`: Emacs patches applied during build
+- `bin/`: Built Go binaries (gitignored)