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_console.py

@@ -0,0 +1,38 @@
+"""Tests for flow.core.console."""
+
+from flow.core.console import Console
+
+
+def test_info_prints_message(capsys):
+    c = Console(color=False)
+    c.info("hello")
+    assert "hello" in capsys.readouterr().out
+
+
+def test_quiet_suppresses_info(capsys):
+    c = Console(quiet=True, color=False)
+    c.info("hidden")
+    assert capsys.readouterr().out == ""
+
+
+def test_quiet_does_not_suppress_error(capsys):
+    c = Console(quiet=True, color=False)
+    c.error("visible")
+    captured = capsys.readouterr()
+    assert "visible" in captured.err or "visible" in captured.out
+
+
+def test_table_prints_headers_and_rows(capsys):
+    c = Console(color=False)
+    c.table(["NAME", "STATUS"], [["foo", "ok"], ["bar", "fail"]])
+    output = capsys.readouterr().out
+    assert "NAME" in output
+    assert "foo" in output
+    assert "bar" in output
+
+
+def test_no_color_strips_ansi(capsys):
+    c = Console(color=False)
+    c.info("test")
+    output = capsys.readouterr().out
+    assert "\033[" not in output