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>

PROGRESS.md

@@ -0,0 +1,38 @@
+# PROGRESS.md
+
+Tracks implementation status. See PROJECT_PLAN.md for the full plan and
+ARCHITECTURE.md for design details.
+
+**If you are picking this up mid-implementation, read this file first.**
+
+## Current Status
+
+**Phase:** Pre-implementation — design complete, ready to begin Step 1.
+
+**Last updated:** 2026-03-23
+
+## Completed Steps
+
+(none yet)
+
+## In Progress
+
+(none yet)
+
+## Up Next
+
+Step 1: Project Scaffolding — remove old C++ files, initialize Go module,
+create directory structure, set up cobra root command.
+
+## Known Issues / Decisions Deferred
+
+- **Blob durability**: blobs are not stored in git. A strategy for backup or
+  replication is deferred to a future phase.
+- **gRPC remote mode**: Phase 2. Package structure is designed to accommodate
+  it (garden core separates logic from CLI wiring).
+
+## Change Log
+
+| Date | Step | Summary |
+|---|---|---|
+| 2026-03-23 | — | Design phase complete. ARCHITECTURE.md and PROJECT_PLAN.md written. |