feat: add nix support

[andreabadesso]

Refer to #8

Summary

• Add flake.nix with packages.default, devShells.default, and overlays.default for Nix-based builds
• Add default.nix via flake-compat for legacy nix-env installation support
• Pin to nixos-25.11 stable channel for production use

Acceptance Criteria

• nix build completes successfully
• nix develop --command python -c "import agentmail" works
• nix-env -if . installs the package (legacy support)
• nix flake check passes
• Dev shell includes pytest, mypy, and ruff
• Pinned to nixos-25.11 stable channel

Usage

# Build the package
nix build

# Enter dev shell (includes pytest, mypy, ruff)
nix develop

# Install via flake
nix profile install github:agentmail-to/agentmail-python