Add reproducible dev environment

- flake.nix: rebrand description, add Go 1.25, gopls, gotools,
  staticcheck, golangci-lint, gnumake to all dev shells. Add a
  plain `dev` shell (`nix develop .#dev`) that does not wrap the
  shell in the bubblewrap sandbox so contributors can use a
  standard Go toolchain.
- Dockerfile.dev: golang:1.22-bookworm with make, git, gopls and
  staticcheck, /workspace as default cwd. CGO disabled.
- README: document both nix and Docker dev paths.

flake.lock is committed for reproducibility.

Closes #6.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -45,10 +45,18 @@ See `./librenotes help` for the full command list.
 ## Development setup
 
 A Nix flake provides a reproducible development environment with Go,
-build tools, and the project CLIs:
+build tools, and the project CLIs. Use the plain `dev` shell for a
+non-sandboxed Go toolchain:
 
 ```sh
-nix develop
+nix develop .#dev
+```
+
+Alternatively, build a Docker-based dev environment:
+
+```sh
+docker build -f Dockerfile.dev -t librenotes-dev .
+docker run --rm -it -v "$PWD:/workspace" librenotes-dev
 ```
 
 The repository layout follows the standard Go project structure: