feat: add Nix flake with home-manager module for declarative builds

[helgeu]

Summary

• Add a Nix flake (flake.nix + nix/package.nix) that builds rtk using rustPlatform.buildRustPackage
• Add --dry-run --json flag to rtk init for declarative config managers (outputs file manifest as JSON)
• Add home-manager module (nix/homeManagerModule.nix) for declarative Claude Code hook integration
• Version is read from Cargo.toml at eval time — no manual version bumps needed
• Includes an overlay (overlays.default) and homeManagerModules.default for Nix consumers

What this enables

# flake.nix
inputs.rtk.url = "github:rtk-ai/rtk";

# home-manager config
imports = [ inputs.rtk.homeManagerModules.default ];
programs.rtk = {
  enable = true;
  claudeCode = {
    enable = true;
    baseClaude = ./CLAUDE.md.template;   # optional: seed CLAUDE.md
    baseSettings = ./settings.json;       # optional: seed settings.json
  };
};

# Run directly without installing
nix run github:rtk-ai/rtk

# Programmatic config generation (for other package managers)
rtk init --dry-run --json

Design decisions

• builtins.fromTOML for version: reads Cargo.toml at eval time so the package version stays in sync with releases automatically
• --dry-run --json: outputs the full file manifest as JSON so Nix (or any declarative tool) can extract files at build time without side effects
• jq -j in module: uses jq -j (not jq -r) to extract file content without adding a trailing newline, preserving exact content for rtk verify integrity checks
• doCheck = false: tests require network access which is unavailable in the Nix sandbox

Test plan

• nix build succeeds on macOS (aarch64)
• rtk verify passes after darwin-rebuild switch with home-manager module
• rtk gain and hook rewriting work end-to-end
• nix build succeeds on Linux (x86_64)

🤖 Generated with Claude Code

[pszymkowiak]

Please retarget this PR to develop instead of master. We use develop as the integration branch — master is for releases only. You can change the base branch in the PR settings. Thanks!

[helgeu]

Sorry about that! Retargeted to develop and rebased. Should be good now.