feat: rewrite core layer (errors, template, paths, platform, console, runtime, config)

Complete rewrite of all core modules with proper abstractions:
- FlowError hierarchy with PlanConflict and ExecutionError
- Pure template substitution ($VAR, ${VAR}, {{expr}})
- XDG path constants
- Frozen PlatformInfo dataclass with context detection
- Console with color/quiet/TTY support
- Runtime primitives (CommandRunner, FileSystem, GitClient, SystemRuntime)
- Config loading with target parsing and manifest merging

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

tests/test_core_config.py

@@ -0,0 +1,77 @@
+"""Tests for flow.core.config."""
+
+from pathlib import Path
+
+import pytest
+
+from flow.core.config import AppConfig, load_config, load_manifest
+
+
+def test_load_config_missing_path(tmp_path):
+    cfg = load_config(tmp_path / "nonexistent")
+    assert isinstance(cfg, AppConfig)
+    assert cfg.dotfiles_url == ""
+    assert cfg.container_registry == "registry.tomastm.com"
+
+
+def test_load_config_from_yaml(tmp_path):
+    (tmp_path / "config.yaml").write_text(
+        "repository:\n"
+        "  url: git@github.com:user/dots.git\n"
+        "  branch: dev\n"
+        "paths:\n"
+        "  projects: ~/code\n"
+        "defaults:\n"
+        "  container-registry: my.registry.com\n"
+        "  tmux-session: main\n"
+    )
+    cfg = load_config(tmp_path)
+    assert cfg.dotfiles_url == "git@github.com:user/dots.git"
+    assert cfg.dotfiles_branch == "dev"
+    assert cfg.projects_dir == "~/code"
+    assert cfg.container_registry == "my.registry.com"
+    assert cfg.tmux_session == "main"
+
+
+def test_load_config_parses_targets_shorthand(tmp_path):
+    (tmp_path / "config.yaml").write_text(
+        "targets:\n"
+        "  personal@orb: personal.orb\n"
+    )
+    cfg = load_config(tmp_path)
+    assert len(cfg.targets) == 1
+    assert cfg.targets[0].namespace == "personal"
+    assert cfg.targets[0].platform == "orb"
+    assert cfg.targets[0].host == "personal.orb"
+
+
+def test_load_config_parses_targets_dict(tmp_path):
+    (tmp_path / "config.yaml").write_text(
+        "targets:\n"
+        "  work@ec2:\n"
+        "    host: work.ec2.internal\n"
+        "    identity: ~/.ssh/id_work\n"
+    )
+    cfg = load_config(tmp_path)
+    assert len(cfg.targets) == 1
+    assert cfg.targets[0].host == "work.ec2.internal"
+    assert cfg.targets[0].identity == "~/.ssh/id_work"
+
+
+def test_load_manifest_returns_dict(tmp_path):
+    (tmp_path / "manifest.yaml").write_text(
+        "packages:\n"
+        "  - name: fd\n"
+        "    type: pkg\n"
+    )
+    data = load_manifest(tmp_path)
+    assert isinstance(data, dict)
+    assert "packages" in data
+
+
+def test_load_manifest_merges_files(tmp_path):
+    (tmp_path / "01-packages.yaml").write_text("packages:\n  - name: fd\n    type: pkg\n")
+    (tmp_path / "02-profiles.yaml").write_text("profiles:\n  work:\n    os: linux\n")
+    data = load_manifest(tmp_path)
+    assert "packages" in data
+    assert "profiles" in data