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.

extension/src/content/markdown/index.ts

@@ -0,0 +1,9 @@
+import type { ContentArgs } from '../../types';
+import { pickMarkdownRoot } from './root';
+import { renderMarkdown } from './renderer';
+import { stripNoise } from './utils';
+
+export function extractMarkdown({ selector }: ContentArgs) {
+  const root = stripNoise(pickMarkdownRoot(selector));
+  return renderMarkdown(root);
+}