nix: add flake + packaging

[Mag1cByt3s]

What this PR does:

Adds Nix support for PHPGGC so it can be built/run fully reproducible using Nix without Docker.

• Adds flake.nix with:
  • packages.phpggc / packages.default (nix build .#phpggc)
  • apps.default (nix run .#phpggc -- …)
  • devShells.default (nix develop)
• Adds package.nix that packages the repository as a Nix derivation:
  • Installs the upstream sources (gadgetchains/, lib/, templates/, etc.) into $out/share/phpggc
  • Provides a phpggc wrapper in $out/bin/phpggc that runs via Nix’s PHP with phar.readonly=0 and changes the cwd into the installed share/ dir so relative includes work
• Adds shell.nix dev environment roughly mirroring the Dockerfile tooling:
  • php, composer, curl, python3, python3Packages.rich, plus the packaged phpggc

Why:

• Makes it trivial to run PHPGGC on Nix/NixOS and provides a reproducible workflow for development/testing.
• Avoids reliance on container images for day-to-day usage.

How to use / test:
From the repo root:

• Build:
  • nix build .#phpggc
• Run:
  • nix run .#phpggc -- -l
  • nix run .#phpggc -- <chain> [args…]
• Dev shell:
  • nix develop
  • (non-flake alternative): nix-shell

Notes:

• The wrapper enforces -d phar.readonly=0 to match the Dockerfile behavior and support PHAR payload generation when requested.
• The packaged phpggc script is stored under $out/share/phpggc (invoked via the wrapper), so it doesn't rely on the source checkout layout at runtime.

Files added:

• flake.nix
• shell.nix
• package.nix
• flake.lock