feat: add Nix flake support for one-command installation

[levonk]

Summary

This issue tracks adding Nix flake support to the upstream project so users can install and run it without cloning or compiling manually.

What Nix provides

• Pure / Hermetic builds: every input — compiler, libraries, system dependencies — is pinned in flake.lock. If it builds today, it builds in ten years.
• Reproducible: the exact same derivation always produces the exact same output bit-for-bit (modulo timestamps). No "works on my machine."
• Idempotent installs: running nix profile install twice is a no-op. The system reaches the declared state and stays there.
• Rollback-able: nix profile rollback restores the previous profile generation instantly. Broken update? One command back.
• Declarative: the entire build is a single expression (flake.nix). No imperative apt install, brew install, make dance.
• Cross-platform: same nix run github:owner/repo works on macOS (Apple Silicon & Intel) and Linux. The flake handles platform-specific dependencies.
• Zero-install runs: nix run fetches from binary cache when available. No clone, no compile, no cargo build.
• No system pollution: nix profile install adds to a user-specific profile. Uninstall cleanly with nix profile remove. No orphaned global packages.
• Atomic upgrades / downgrades: profiles are switched atomically. No half-upgraded state.

Current gap

The project currently only documents source builds (npm install). There is no one-command install path for users who already have Nix.

Proposed change

• Add flake.nix with packages.default and apps.default
• Add devbox.json for reproducible development environments
• Update README install section to include Nix (flakes) and Devbox instructions

Branch

feat-nix-package-manager-install on the fork.

Implementation

I have prepared the implementation in my fork at:
https://github.com/levonk/space-agent/tree/feat-nix-package-manager-install

The changes include:

• flake.nix: Nix flake with packages.default and apps.default using Node.js 22 (Node.js 20 is marked as insecure)
• devbox.json: Devbox configuration for reproducible development environments
• README.md: added "With Nix (flakes)" and "With Devbox" install subsections

Tested locally with nix build . and binary execution verification.