refactor: modularize auth transport and markdown

- Split auth into focused package modules for agent keys, file keys,
  signing, and post-quantum transport helpers while keeping the public
  browser_cli.auth import surface intact.
- Move transport encoding internals into a package with separate codec and
  binary-hoisting helpers, preserving browser_cli.transport compatibility.
- Extract remote TCP auth/socket helpers and serve challenge setup out of the
  runtime paths to make connection handling easier to reason about.
- Move the extension markdown extractor into a dedicated content/markdown
  folder with separate root selection, code normalization, renderer, and utils.
- Centralize CLI Rich rendering helpers for tab/window tree and table output,
  and add rendering tests for the shared builders.
- Remove local typing ignores in SDK/decorator/script plumbing and bump the
  package and extension version to 0.15.3.

browser_cli/sdk/base.py

@@ -9,7 +9,7 @@ from __future__ import annotations
 
 from collections.abc import Callable
 from functools import wraps
-from typing import Any, TypeVar
+from typing import Any, TypeVar, cast
 
 F = TypeVar("F", bound=Callable)
 _MISSING = object()
@@ -54,8 +54,8 @@ def sdk_command(
         return _clone_default(default)
       return result
 
-    wrapper._browser_cli_command = name  # type: ignore[attr-defined]
-    return wrapper  # type: ignore[return-value]
+    setattr(wrapper, "_browser_cli_command", name)
+    return cast(F, wrapper)
 
   return decorator