docs(agents): note that blueprint package discovery requires git staging

Blueprint reads packages from the git index, not the working directory.
New packages must be staged with 'git add' before they appear in flake
outputs. Add this to both the Blueprint Framework section and the
'Adding a New Package' workflow.

AGENTS.md

@@ -23,6 +23,8 @@ This project uses `numtide/blueprint` which:
 - Handles formatting, checking, and devShell generation
 - Exposes `mkPackagesFor` function to build packages against any nixpkgs instance
 
+**Important**: Blueprint's package discovery reads from the git index, not the working directory. New package directories must be staged with `git add` before they appear in flake outputs. Unstaged files are invisible to `nix build`, `nix flake show`, and `nix run`.
+
 ### Two Overlay Strategies
 
 1. **`overlays.default`** - Binary-cache-friendly
@@ -159,6 +161,10 @@ When adding a new package:
    pkgs.callPackage ./package.nix { }
    ```
 
+3.5. **Stage the package**: `git add packages/<package-name>/`
+   Blueprint discovers packages from the git index, not the working directory.
+   Without this step, the package won't appear in flake outputs.
+
 4. **Test the package**:
    ```bash
    nix build .#<package-name>