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_parsers/test_action.py

@@ -71,22 +71,28 @@ async def test_action_with_nargs_positional():
         return int(a) * int(b)
 
     action = Action("multiply", multiply)
-    parser.add_argument("mul", action=ArgumentAction.ACTION, resolver=action, nargs=2)
+    parser.add_argument(
+        "mul",
+        action=ArgumentAction.ACTION,
+        resolver=action,
+        nargs=2,
+        type=int,
+    )
     args = await parser.parse_args(["3", "4"])
     assert args["mul"] == 12
 
     with pytest.raises(CommandArgumentError):
         await parser.parse_args(["3"])
 
-    with pytest.raises(CommandArgumentError):
-        await parser.parse_args([])
-
     with pytest.raises(CommandArgumentError):
         await parser.parse_args(["3", "4", "5"])
 
     with pytest.raises(CommandArgumentError):
         await parser.parse_args(["--mul", "3", "4"])
 
+    with pytest.raises(CommandArgumentError):
+        await parser.parse_args([])
+
 
 @pytest.mark.asyncio
 async def test_action_with_nargs_positional_int():
@@ -102,6 +108,9 @@ async def test_action_with_nargs_positional_int():
     args = await parser.parse_args(["3", "4"])
     assert args["mul"] == 12
 
+    with pytest.raises(CommandArgumentError):
+        await parser.parse_args([])
+
     with pytest.raises(CommandArgumentError):
         await parser.parse_args(["3"])
 
@@ -209,11 +218,19 @@ async def test_action_with_default_and_value_not():
 @pytest.mark.asyncio
 async def test_action_with_default_and_value_positional():
     parser = CommandArgumentParser()
-    action = Action("default", lambda: "default_value")
-    parser.add_argument("default", action=ArgumentAction.ACTION, resolver=action)
+    action = Action("action", lambda x: x)
+    parser.add_argument(
+        "default",
+        action=ArgumentAction.ACTION,
+        resolver=action,
+        default="default_value",
+    )
+
+    args = await parser.parse_args([])
+    assert args["default"] == "default_value"
+
+    args = await parser.parse_args(["be"])
+    assert args["default"] == "be"
 
     with pytest.raises(CommandArgumentError):
-        await parser.parse_args([])
-
-    with pytest.raises(CommandArgumentError):
-        await parser.parse_args(["be"])
+        await parser.parse_args(["one", "new_value"])