Add design docs for Go rewrite.

ARCHITECTURE.md: tech stack (Go, YAML, cobra), repo layout, manifest
schema, content-addressable blob store, CLI commands, package structure.
PROJECT_PLAN.md: 8-step implementation sequence with parallelism notes.
PROGRESS.md: status tracker for resumable implementation.
CLAUDE.md: project guidance for Claude Code, references design docs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,54 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Critical: Keep Project Docs Updated
+
+Any change to the codebase MUST be reflected in these files:
+
+- **ARCHITECTURE.md** — design decisions, data model, package structure
+- **PROJECT_PLAN.md** — implementation steps; check off completed items
+- **PROGRESS.md** — current status, change log; update after completing any step
+
+If another agent or engineer picks this up later, these files are how they
+resume. Keeping them accurate is not optional.
+
+## Project
+
+sgard (Shimmering Clarity Gardener) — a dotfiles manager.
+Module: `github.com/kisom/sgard`. Author: K. Isom <kyle@imap.cc>.
+
+## Build
+
+```bash
+go build ./cmd/sgard
+```
+
+Run tests:
+```bash
+go test ./...
+```
+
+## Dependencies
+
+- `gopkg.in/yaml.v3` — manifest serialization
+- `github.com/spf13/cobra` — CLI framework
+
+## Package Structure
+
+```
+cmd/sgard/    CLI entry point (cobra commands, pure wiring)
+garden/       Core business logic (Garden struct orchestrating everything)
+manifest/     YAML manifest parsing (Manifest/Entry structs, Load/Save)
+store/        Content-addressable blob storage (SHA-256 keyed)
+```
+
+Key rule: all logic lives in `garden/`. The `cmd/` layer only parses flags
+and calls `Garden` methods. This enables the future gRPC server to reuse
+the same logic with zero duplication.
+
+## Legacy Files
+
+Old C++ and proto source files may still be present. They are retained in
+git history for reference and should be removed as part of the Go rewrite
+(see PROJECT_PLAN.md Step 1).