Resolves #324 — the curl|sh install scripts don't work under Nix
(FHS paths like ~/.local/bin aren't on $PATH).

The flake provides:
- packages.default: codegraph built with buildNpmPackage (Node.js 24)
- devShells.default: development shell with Node.js 24

The wrapper injects --liftoff-only to prevent the V8 turboshaft WASM
Zone OOM (issues #293/#298), matching the existing bundle launcher.

Supports linux and darwin on both x86_64 and aarch64.

…tory imports

- Ensure fileExists validates path is a regular file with fs.statSync().isFile()
- Validate target node exists in getNodeById() before resolving file-level imports
- Filter out non-existent target nodes in createEdges() to guarantee SQLite foreign key integrity
- Add unit test for Swift test imports matching root directory names