feat(core): centralize command execution and add standalone command runner

- add CommandExecutor to unify shared command execution lifecycle
  across Falyx and standalone command execution
- add CommandRunner for running a single Command directly as a CLI
  or programmatic entrypoint
- add Command.build() factory and rename parse_args() to resolve_args()
  to clarify the parsing-to-execution boundary
- introduce ExecutionOption and wire execution-scoped flags into
  CommandArgumentParser and Command construction
- refactor Falyx to use FalyxParser/ParseResult and CommandExecutor
  instead of the older argparse-based flow and run_key path
- simplify __main__.py bootstrap by building a bootstrap Falyx instance
  directly and running flx.run()
- improve completer support for preview commands and unique-prefix
  command resolution
- default BottomBar toggle namespace to "default"
- expand module/class docstrings to reflect the new execution architecture

tests/test_execute_command.py

@@ -0,0 +1,46 @@
+import pytest
+
+from falyx import Falyx
+from falyx.action import Action
+
+
+@pytest.mark.asyncio
+async def test_execute_command():
+    """Test if Falyx can run in run key mode."""
+    falyx = Falyx("Run Key Test")
+
+    # Add a simple command
+    falyx.add_command(
+        key="T",
+        description="Test Command",
+        action=lambda: "Hello, World!",
+    )
+
+    # Run the CLI
+    result = await falyx.execute_command("T")
+    assert result == "Hello, World!"
+
+
+@pytest.mark.asyncio
+async def test_execute_command_recover():
+    """Test if Falyx can recover from a failure in run key mode."""
+    falyx = Falyx("Run Key Recovery Test")
+
+    state = {"count": 0}
+
+    async def flaky():
+        if not state["count"]:
+            state["count"] += 1
+            raise RuntimeError("Random failure!")
+        return "ok"
+
+    # Add a command that raises an exception
+    falyx.add_command(
+        key="E",
+        description="Error Command",
+        action=Action("flaky", flaky),
+        retry=True,
+    )
+
+    result = await falyx.execute_command("E")
+    assert result == "ok"