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_planning.py

@@ -0,0 +1,64 @@
+"""Tests for package install/remove planning."""
+
+from pathlib import Path
+
+from flow.domain.packages.models import (
+    InstalledPackage,
+    InstalledState,
+    PackageDef,
+)
+from flow.domain.packages.planning import plan_install, plan_remove
+
+
+def _pkg(name, type="pkg", sources=None, source=None, version=None,
+         asset_pattern=None, platform_map=None):
+    return PackageDef(
+        name=name, type=type, sources=sources or {},
+        source=source, version=version, asset_pattern=asset_pattern,
+        platform_map=platform_map or {}, extract_dir=None, install={},
+        post_install=None, allow_sudo=False,
+    )
+
+
+class TestPlanInstall:
+    def test_new_pm_package(self):
+        pkgs = [_pkg("fd", sources={"apt": "fd-find"})]
+        plan = plan_install(pkgs, InstalledState(), "linux-x64", pm="apt")
+        assert len(plan.install_ops) == 1
+        assert plan.install_ops[0].method == "pm"
+        assert plan.install_ops[0].source_name == "fd-find"
+        assert plan.pm_update_needed is True
+
+    def test_skip_already_installed(self):
+        pkgs = [_pkg("fd")]
+        installed = InstalledState(packages={
+            "fd": InstalledPackage(name="fd", version="1.0", type="pkg"),
+        })
+        plan = plan_install(pkgs, installed, "linux-x64", pm="apt")
+        assert len(plan.install_ops) == 0
+
+    def test_binary_package(self):
+        pkgs = [_pkg("nvim", type="binary", source="github:neovim/neovim",
+                      version="v0.10.4",
+                      platform_map={"linux-x64": "nvim-linux-x86_64.tar.gz"})]
+        plan = plan_install(pkgs, InstalledState(), "linux-x64", pm="apt")
+        assert len(plan.install_ops) == 1
+        assert plan.install_ops[0].method == "binary"
+        assert plan.install_ops[0].download_url is not None
+
+
+class TestPlanRemove:
+    def test_remove_installed(self):
+        installed = InstalledState(packages={
+            "fd": InstalledPackage(
+                name="fd", version="1.0", type="pkg",
+                files=[Path("/usr/bin/fd")],
+            ),
+        })
+        plan = plan_remove(["fd"], installed)
+        assert len(plan.remove_ops) == 1
+        assert plan.remove_ops[0].name == "fd"
+
+    def test_remove_not_installed(self):
+        plan = plan_remove(["missing"], InstalledState())
+        assert len(plan.remove_ops) == 0