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_platform.py

@@ -0,0 +1,38 @@
+"""Tests for flow.core.platform."""
+
+import os
+
+import pytest
+
+from flow.core.platform import PlatformInfo, detect_context, detect_platform
+
+
+def test_platform_info_computes_platform_string():
+    p = PlatformInfo(os="linux", arch="x64")
+    assert p.platform == "linux-x64"
+
+
+def test_detect_platform_returns_valid_info():
+    info = detect_platform()
+    assert info.os in ("linux", "macos")
+    assert info.arch in ("x64", "arm64")
+    assert info.platform == f"{info.os}-{info.arch}"
+
+
+def test_detect_platform_raises_flow_error_on_unsupported(monkeypatch):
+    from flow.core.errors import FlowError
+    monkeypatch.setattr("platform.system", lambda: "FreeBSD")
+    with pytest.raises(FlowError, match="Unsupported operating system"):
+        detect_platform()
+
+
+def test_detect_context_host(monkeypatch):
+    monkeypatch.delenv("DF_NAMESPACE", raising=False)
+    monkeypatch.delenv("DF_PLATFORM", raising=False)
+    assert detect_context() == "host"
+
+
+def test_detect_context_vm(monkeypatch):
+    monkeypatch.setenv("DF_NAMESPACE", "personal")
+    monkeypatch.setenv("DF_PLATFORM", "orb")
+    assert detect_context() == "vm"