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_runtime.py

@@ -0,0 +1,95 @@
+"""Tests for flow.core.runtime."""
+
+from pathlib import Path
+
+from flow.core.runtime import CommandRunner, FileSystem, GitClient, SystemRuntime
+
+
+class TestFileSystem:
+    def test_ensure_dir_creates_nested(self, tmp_path):
+        fs = FileSystem()
+        target = tmp_path / "a" / "b" / "c"
+        fs.ensure_dir(target)
+        assert target.is_dir()
+
+    def test_write_and_read_text(self, tmp_path):
+        fs = FileSystem()
+        path = tmp_path / "test.txt"
+        fs.write_text(path, "hello")
+        assert fs.read_text(path) == "hello"
+
+    def test_read_text_default(self, tmp_path):
+        fs = FileSystem()
+        path = tmp_path / "missing.txt"
+        assert fs.read_text(path, default="fallback") == "fallback"
+
+    def test_write_and_read_json(self, tmp_path):
+        fs = FileSystem()
+        path = tmp_path / "data.json"
+        fs.write_json(path, {"key": "value"})
+        assert fs.read_json(path) == {"key": "value"}
+
+    def test_create_symlink(self, tmp_path):
+        fs = FileSystem()
+        source = tmp_path / "source"
+        source.write_text("content")
+        target = tmp_path / "link"
+        fs.create_symlink(source, target)
+        assert target.is_symlink()
+        assert target.resolve() == source.resolve()
+
+    def test_same_symlink_true(self, tmp_path):
+        fs = FileSystem()
+        source = tmp_path / "source"
+        source.write_text("content")
+        target = tmp_path / "link"
+        target.symlink_to(source)
+        assert fs.same_symlink(target, source) is True
+
+    def test_same_symlink_false(self, tmp_path):
+        fs = FileSystem()
+        source = tmp_path / "source"
+        source.write_text("content")
+        other = tmp_path / "other"
+        other.write_text("other")
+        target = tmp_path / "link"
+        target.symlink_to(other)
+        assert fs.same_symlink(target, source) is False
+
+    def test_remove_file(self, tmp_path):
+        fs = FileSystem()
+        path = tmp_path / "file"
+        path.write_text("x")
+        fs.remove_file(path)
+        assert not path.exists()
+
+    def test_remove_file_missing_ok(self, tmp_path):
+        fs = FileSystem()
+        fs.remove_file(tmp_path / "missing", missing_ok=True)  # no error
+
+    def test_copy_file(self, tmp_path):
+        fs = FileSystem()
+        src = tmp_path / "src"
+        src.write_text("data")
+        dst = tmp_path / "sub" / "dst"
+        fs.copy_file(src, dst)
+        assert dst.read_text() == "data"
+
+
+class TestCommandRunner:
+    def test_run_echo(self):
+        runner = CommandRunner()
+        result = runner.run(["echo", "hello"], capture_output=True)
+        assert result.stdout.strip() == "hello"
+
+    def test_require_binary_finds_echo(self):
+        runner = CommandRunner()
+        path = runner.require_binary("echo")
+        assert path is not None
+
+
+class TestSystemRuntime:
+    def test_creates_git_client(self):
+        rt = SystemRuntime()
+        assert isinstance(rt.git, GitClient)
+        assert rt.git.runner is rt.runner