feat: add all domain layers (dotfiles, packages, remote, containers)

- Dotfiles: models, module resolution, path resolution, link planning
- Packages: models, catalog parsing, resolution, install/remove planning
- Remote: target parsing, SSH command building
- Containers: image refs, mount resolution, container specs

All domain code is pure functions + frozen dataclasses. 88 tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test_domain_packages_models.py

@@ -0,0 +1,43 @@
+"""Tests for packages domain models."""
+
+from pathlib import Path
+
+import pytest
+
+from flow.core.errors import ConfigError
+from flow.domain.packages.models import InstalledPackage, InstalledState, PackageDef
+
+
+def test_installed_state_roundtrip():
+    state = InstalledState(packages={
+        "neovim": InstalledPackage(
+            name="neovim", version="0.10.4", type="binary",
+            files=[Path("/home/x/.local/bin/nvim")],
+        ),
+    })
+    data = state.as_dict()
+    restored = InstalledState.from_dict(data)
+    assert "neovim" in restored.packages
+    assert restored.packages["neovim"].version == "0.10.4"
+    assert restored.packages["neovim"].files == [Path("/home/x/.local/bin/nvim")]
+
+
+def test_installed_state_empty():
+    state = InstalledState.from_dict({})
+    assert state.packages == {}
+
+
+def test_installed_state_version_mismatch():
+    with pytest.raises(ConfigError):
+        InstalledState.from_dict({"version": 99, "packages": {}})
+
+
+def test_package_def_fields():
+    pkg = PackageDef(
+        name="fd", type="pkg", sources={"apt": "fd-find"},
+        source=None, version=None, asset_pattern=None,
+        platform_map={}, extract_dir=None, install={},
+        post_install=None, allow_sudo=False,
+    )
+    assert pkg.name == "fd"
+    assert pkg.type == "pkg"