docs(installation): update Nix flake guide to use overlay pattern

The CLI now exports overlays.default, enabling users to integrate probitas
into their pkgs namespace rather than referencing it via packages.${system}.
This provides cleaner configuration with 'with pkgs;' syntax and ensures
consistent Deno versions by using the user's nixpkgs automatically.

Added "Why Use Overlays?" section explaining the benefits of unified
namespace, composability, and automatic Deno version consistency.

Updated Flake Outputs Reference table to include overlays.default.

Author: lambdalisue

docs/installation.md

@@ -63,8 +63,7 @@ nix profile install github:jsr-probitas/cli
 
 ### Add to Project's flake.nix
 
-Add Probitas to your project's development environment. Create or update
-`flake.nix`:
+Add Probitas to your project's development environment using the overlay:
 
 ```nix
 {
@@ -79,12 +78,15 @@ Add Probitas to your project's development environment. Create or update
   outputs = { self, nixpkgs, flake-utils, probitas }:
     flake-utils.lib.eachDefaultSystem (system:
       let
-        pkgs = import nixpkgs { inherit system; };
+        pkgs = import nixpkgs {
+          inherit system;
+          overlays = [ probitas.overlays.default ];
+        };
       in {
         devShells.default = pkgs.mkShell {
-          packages = [
-            pkgs.deno
-            probitas.packages.${system}.default
+          packages = with pkgs; [
+            deno
+            probitas
           ];
         };
       });
@@ -97,6 +99,23 @@ Enter the development environment:
 nix develop
 ```
 
+### Why Use Overlays?
+
+The overlay pattern integrates `probitas` directly into your `pkgs`, enabling
+cleaner configuration:
+
+```nix
+overlays = [ probitas.overlays.default ];
+# ...
+packages = with pkgs; [ deno probitas ];  # probitas is now part of pkgs
+```
+
+Benefits:
+
+- **Unified namespace**: Access `probitas` like any nixpkgs package
+- **Composable**: Combine with other overlays seamlessly
+- **Consistent Deno**: Uses your project's nixpkgs Deno version automatically
+
 ### Why Use inputs.follows?
 
 The `inputs.follows` directive ensures Probitas uses your project's nixpkgs
@@ -132,12 +151,13 @@ probitas.url = "github:jsr-probitas/cli/abc1234";
 
 The Probitas CLI flake provides:
 
-| Output                        | Description                 |
-| ----------------------------- | --------------------------- |
-| `packages.${system}.default`  | The `probitas` CLI package  |
-| `packages.${system}.probitas` | Alias for the CLI package   |
-| `apps.${system}.default`      | App for `nix run`           |
-| `devShells.${system}.default` | Development shell with Deno |
+| Output                        | Description                       |
+| ----------------------------- | --------------------------------- |
+| `overlays.default`            | Overlay adding `probitas` to pkgs |
+| `packages.${system}.default`  | The `probitas` CLI package        |
+| `packages.${system}.probitas` | Alias for the CLI package         |
+| `apps.${system}.default`      | App for `nix run`                 |
+| `devShells.${system}.default` | Development shell with Deno       |
 
 ## Verify Installation