feat: migrate to flake-parts + devenv, replace attic with nix-bindings

[ProjectInitiative]

Complete development environment consolidation:

Flake restructure:

• Replace flake-utils with flake-parts + devenv.flakeModule
• Add devenv.nix with declarative languages.rust, tools, and config generation
• Add nixConfig for devenv.cachix.org binary cache
• Support 4 systems (x86_64/aarch64 linux/darwin)
• Reduce flake.nix from 310 to 120 lines

Remove attic dependency (broken C++ bindings with nix 2.31+):

• Drop attic-flake flake input and attic crate from Cargo.toml
• Vendor nix-bindings crate for C API store operations
• Add src/nix_store.rs - CLI-backed store ops (nix path-info, nar dump-path, nix-store --query)
• Add src/hash.rs - Nix hash types from existing deps (sha2, nix-base32)
• Add src/signing.rs - NixKeypair from existing dep (ed25519-compact)
• Remove nix version pinning from crane.nix

Dev environment:

• Add treefmt.toml (nixfmt, rustfmt, shfmt, prettier)
• Add AGENTS.md with architecture docs and available commands
• Add devenv.lock + devenv.yaml for devenv 2.x input tracking
• Update .envrc to use devenv
• Update .gitignore for .devenv/
• Disable cachix.enable in devenv.nix to respect system nix config

Testing:

• All integration tests pass (NixOS VM test with Garage S3)
• nix flake check passes on all systems
• nix build produces working binary
• Fixed narHash parsing to handle nix path-info --json base64 format

TODO: Replace CLI calls (query_path_info, nar_from_path) with nix C API when nix_api_store.h exposes those functions.