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_errors.py

@@ -0,0 +1,21 @@
+"""Tests for flow.core.errors."""
+
+from flow.core.errors import ConfigError, ExecutionError, FlowError, PlanConflict
+
+
+def test_flow_error_is_exception():
+    assert issubclass(FlowError, Exception)
+
+
+def test_config_error_is_flow_error():
+    assert issubclass(ConfigError, FlowError)
+
+
+def test_plan_conflict_carries_conflicts():
+    err = PlanConflict("2 conflicts", ["a exists", "b exists"])
+    assert str(err) == "2 conflicts"
+    assert err.conflicts == ["a exists", "b exists"]
+
+
+def test_execution_error_is_flow_error():
+    assert issubclass(ExecutionError, FlowError)