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/markdown/html.py

@@ -4,26 +4,6 @@ from __future__ import annotations
 import re
 from html.parser import HTMLParser
 
-def _normalize_text(value):
-  return re.sub(r"\s+", " ", value or "").strip()
-
-def _normalize_inline(value):
-  value = value.replace("\xa0", " ")
-  value = re.sub(r"[ \t\r\f\v]+", " ", value)
-  value = re.sub(r" *\n *", "\n", value)
-  return value.strip()
-
-def _collapse_blank_lines(value):
-  value = re.sub(r"[ \t]+\n", "\n", value)
-  value = re.sub(r"\n{3,}", "\n\n", value)
-  return value.strip()
-
-def _escape_markdown(text):
-  return re.sub(r"([\\`[\]])", r"\\\1", text)
-
-def _escape_table_cell(text):
-  return text.replace("|", r"\|").replace("\n", " ").strip()
-
 class _HtmlNode:
   def __init__(self, tag=None, attrs=None, text=None):
     self.tag = tag