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>

README.md

@@ -28,6 +28,15 @@ Or install into `$GOBIN`:
 go install github.com/kisom/sgard/cmd/sgard@latest
 ```
 
+NixOS (flake):
+
+```
+nix profile install github:kisom/sgard
+```
+
+Or add to your flake inputs and include `sgard.packages.${system}.default`
+in your packages.
+
 Binaries are also available on the
 [releases page](https://github.com/kisom/sgard/releases).