chore: update Nix shell prompt configuration

Replace PS1 export with NIX_PROJECT_SHELL environment variable for
better integration with shell prompt configurations.

Also add trailing newline to flake.nix for proper formatting.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

flake.nix

@@ -48,8 +48,8 @@
             fi
 
             # Prompt tweak so you can see when venv is active
-            export PS1="(SPARC-venv) $PS1"
+            export NIX_PROJECT_SHELL="SPARC"
           '';
         };
       });
-}
+}