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/decorators.py

@@ -5,7 +5,7 @@ import asyncio
 import functools
 import inspect
 from collections.abc import Callable
-from typing import TypeVar
+from typing import TypeVar, cast
 
 from browser_cli.sdk.base import Namespace
 from browser_cli.sdk.workflow_decorators import WorkflowDecoratorsMixin, _NO_INJECT
@@ -53,7 +53,7 @@ class DecoratorsNS(WorkflowDecoratorsMixin, Namespace):
           finally:
             if cleanup is not None:
               await asyncio.to_thread(cleanup, value)
-        return async_wrapper  # type: ignore[return-value]
+        return cast(F, async_wrapper)
       return WorkflowDecoratorsMixin._value_decorator(
         self, fn, get_value, keyword=keyword, cleanup=cleanup
       )
@@ -74,7 +74,7 @@ class DecoratorsNS(WorkflowDecoratorsMixin, Namespace):
           finally:
             if previous:
               await asyncio.to_thread(self._c.perf.set_profile, previous)
-        return async_wrapper  # type: ignore[return-value]
+        return cast(F, async_wrapper)
       return WorkflowDecoratorsMixin.performance_profile(self, profile, restore=restore)(fn)
     return decorator
 
@@ -101,7 +101,7 @@ class DecoratorsNS(WorkflowDecoratorsMixin, Namespace):
                 raise
               if delay > 0:
                 await asyncio.sleep(delay)
-          raise last_error  # type: ignore[misc]
-        return async_wrapper  # type: ignore[return-value]
+          raise cast(BaseException, last_error)
+        return cast(F, async_wrapper)
       return WorkflowDecoratorsMixin.retry(self, times=times, delay=delay, exceptions=exceptions)(fn)
     return decorator