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/async_sdk.py

@@ -12,7 +12,7 @@ from __future__ import annotations
 import asyncio
 import functools
 from collections.abc import Callable
-from typing import TypeVar
+from typing import TypeVar, cast
 
 from browser_cli.models import Group, Tab
 from browser_cli.sdk import NAMESPACE_NAMES
@@ -74,7 +74,7 @@ class AsyncDecoratorsNS(WorkflowDecoratorsMixin):
         finally:
           if cleanup is not None:
             await self._maybe_await(cleanup(value))
-      return wrapper  # type: ignore[return-value]
+      return cast(F, wrapper)
     return decorator(func) if func is not None else decorator
 
   def new_tab(
@@ -117,7 +117,7 @@ class AsyncDecoratorsNS(WorkflowDecoratorsMixin):
         finally:
           if previous:
             await self._c.perf.set_profile(previous)
-      return wrapper  # type: ignore[return-value]
+      return cast(F, wrapper)
     return decorator
 
   def retry(
@@ -142,8 +142,8 @@ class AsyncDecoratorsNS(WorkflowDecoratorsMixin):
               raise
             if delay > 0:
               await asyncio.sleep(delay)
-        raise last_error  # type: ignore[misc]
-      return wrapper  # type: ignore[return-value]
+        raise cast(BaseException, last_error)
+      return cast(F, wrapper)
     return decorator
 
 class AsyncBrowserCLI: