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_falyx/test_run.py

@@ -3,17 +3,14 @@ import sys
 import pytest
 
 from falyx import Falyx
-from falyx.parser import get_arg_parsers
 
 
 @pytest.mark.asyncio
 async def test_run_basic(capsys):
-    sys.argv = ["falyx", "run", "-h"]
-    falyx_parsers = get_arg_parsers()
-    assert falyx_parsers is not None, "Falyx parsers should be initialized"
+    sys.argv = ["falyx", "-h"]
     flx = Falyx()
     with pytest.raises(SystemExit):
-        await flx.run(falyx_parsers)
+        await flx.run()
 
     captured = capsys.readouterr()
-    assert "Run a command by its key or alias." in captured.out
+    assert "Show this help menu." in captured.out