Add Nix flake

[subtleGradient]

Add Nix flake and contributor automation.

Summary

• document the automation guidelines in AGENTS.md so future helpers know the repository conventions up front
• add .envrc to auto-load the Nix flake for contributors who opt into direnv
• ship a flake.nix + flake.lock pairing that builds libansilove and exposes a dev shell with clang and the right debugger per platform
• extend .gitignore so the flake's result* outputs and direnv state never pollute commits

Why keep the flake in-tree?

• Streamlined onboarding: README already calls out cross-platform dependency setup (BSDs, Linux, macOS); the flake captures those packages once so new contributors land in a working shell instantly, with pkg-config, clang tools, and GD pre-wired.
• Nix packaging parity: libansilove is in nixpkgs today. This flake lets us validate packaging updates locally without leaving the repo, which reduces churn for the maintainer shepherding upstream bumps.
• Reproducible tooling: Maintainers like @fcambus who juggle multiple projects can rely on Nix to supply consistent compilers, lint tooling, and GD even on Apple Silicon—no conflicts with Homebrew or system packages.
• Optional, zero-risk: Nothing in the CMake flow changes. Builders who do not use Nix continue with the documented mkdir build && cmake .. && make path; the flake is additive and kept tidy via .gitignore.

Testing

• nix develop --command echo ok
• nix build
• nix flake check --all-systems

[Copilot]

Pull Request Overview

This PR adds Nix development tooling to streamline contributor onboarding and provide reproducible build environments. The changes introduce a complete Nix flake configuration that packages libansilove and provides a development shell with all necessary dependencies.

Key changes:

• Adds Nix flake configuration for building libansilove and providing development environment
• Documents repository conventions and automation guidelines for contributors
• Provides direnv integration for automatic environment loading

Reviewed Changes

Copilot reviewed 4 out of 6 changed files in this pull request and generated 2 comments.

File	Description
flake.nix	Complete Nix flake with package definition and dev shell configuration
AGENTS.md	Comprehensive repository guidelines covering build, test, coding style, and PR conventions
CLAUDE.md	Single-line file containing "AGENTS.md"
.envrc	Direnv configuration to auto-load the Nix flake

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[Copilot]

The version is hardcoded in the flake. Consider extracting this from CMakeLists.txt or another authoritative source to avoid version drift between the build system and the Nix package.

Suggested change

	version = "1.4.2";
	# Extract version from CMakeLists.txt
	version = let
	cmakeContents = builtins.readFile ./CMakeLists.txt;
	matches = builtins.match ''set *\( *PROJECT_VERSION +([0-9]+\.[0-9]+\.[0-9]+) *\)'' cmakeContents;
	in if matches != null && matches != [] then builtins.elemAt matches 0 else throw "Could not extract version from CMakeLists.txt";