Update docs for v1.0.0.

ARCHITECTURE.md: update package structure to reflect actual file layout
(per-operation files, version command, flake, goreleaser), fix Garden
struct (clock field, Restore confirm callback, List method), add
.gitignore to repo layout, remove stale C++ note.

README.md: add NixOS installation instructions.
CLAUDE.md: add nix build command.
PROGRESS.md: add post-Step-8 release work to change log.

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

CLAUDE.md

@@ -24,6 +24,11 @@ Module: `github.com/kisom/sgard`. Author: K. Isom <kyle@imap.cc>.
 go build ./cmd/sgard
 ```
 
+Nix:
+```bash
+nix build .#sgard
+```
+
 Run tests:
 ```bash
 go test ./...