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_command_argument_parser.py

@@ -431,7 +431,6 @@ async def test_parse_args_flagged_nargs_plus():
     assert args["files"] == ["a", "b", "c"]
 
     args = await parser.parse_args(["--files", "a"])
-    print(args)
     assert args["files"] == ["a"]
 
     args = await parser.parse_args([])
@@ -666,7 +665,7 @@ async def test_parse_args_split_order():
     cap.add_argument("a")
     cap.add_argument("--x")
     cap.add_argument("b", nargs="*")
-    args, kwargs = await cap.parse_args_split(["1", "--x", "100", "2"])
+    args, kwargs, _ = await cap.parse_args_split(["1", "--x", "100", "2"])
     assert args == ("1", ["2"])
     assert kwargs == {"x": "100"}