docs: add Nix installation and development instructions

Document how to use Nix flake for installation (nix run, nix profile
install) and development (nix develop). Reorganize Installation and
Development sections with clear subsections.

Author: lambdalisue

README.md

@@ -6,6 +6,8 @@ Command-line interface for
 
 ## Installation
 
+### Using install script
+
 ```bash
 curl -fsSL https://raw.githubusercontent.com/jsr-probitas/cli/main/install.sh | bash
 ```
@@ -20,6 +22,23 @@ curl -fsSL https://raw.githubusercontent.com/jsr-probitas/cli/main/install.sh |
 curl -fsSL https://raw.githubusercontent.com/jsr-probitas/cli/main/install.sh | PROBITAS_INSTALL_DIR=/usr/local/bin bash
 ```
 
+### Using Nix
+
+With [Nix](https://nixos.org/) and flakes enabled:
+
+```bash
+# Run directly without installing
+nix run github:jsr-probitas/cli
+
+# Install to profile
+nix profile install github:jsr-probitas/cli
+
+# Use in a flake (flake.nix)
+{
+  inputs.probitas-cli.url = "github:jsr-probitas/cli";
+}
+```
+
 ## Usage
 
 ```bash
@@ -99,6 +118,20 @@ Create a `probitas.json` file in your project root:
 
 ## Development
 
+### Using Nix (recommended)
+
+```bash
+# Enter development shell with all dependencies
+nix develop
+
+# Or run commands directly
+nix develop -c deno task test
+```
+
+### Without Nix
+
+Requires [Deno](https://deno.land/) v2.x or later.
+
 ```bash
 # Run tests
 deno task test