feat: add Nix flake for native Nix/NixOS installation

[uxtechie]

Summary

Resolves #324 — the curl|sh install scripts don't work under Nix (FHS paths like ~/.local/bin aren't on $PATH, and Nix's immutable store is incompatible with imperative installers).

This adds a flake.nix so Nix/NixOS users can install codegraph natively.

Changes

File	Description
flake.nix	Nix flake with buildNpmPackage derivation + dev shell
flake.lock	Auto-generated lock file (nixpkgs-unstable + flake-utils)
.gitignore	Added result (Nix build output symlink)

Details

• Uses Node.js 24 (matching build-bundle.sh's pinned v24.16.0)
• Wrapper injects --liftoff-only to prevent the V8 turboshaft WASM Zone OOM (issues Fatal process out of memory: Zone #293/Fatal process out of memory (Zone) during parsing — V8 WASM compilation OOM on Node v24 #298), matching the existing bundle launcher
• Supports linux and darwin on both x86_64 and aarch64
• No native addons — pure JS + WASM, so builds on any platform without cross-compilation
• Includes a devShells.default for contributors

Usage

# Run directly
nix run github:colbymchenry/codegraph

# Install to profile
nix profile install github:colbymchenry/codegraph

# Use in another flake / NixOS config
inputs.codegraph.url = "github:colbymchenry/codegraph";

# Development shell
nix develop github:colbymchenry/codegraph

Verification

• nix build .#default — builds successfully
• ./result/bin/codegraph --version → 0.9.3
• ./result/bin/codegraph --help → full command list
• --liftoff-only present in wrapper script
• nix flake check — all systems evaluate cleanly

Note for future releases

When package-lock.json changes, the npmDepsHash in flake.nix must be updated. Workflow: set hash to "", run nix build, copy the correct hash from the error.