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.

tests/test_rendering.py

@@ -3,6 +3,7 @@ from os import terminal_size
 from rich.console import Console
 from rich.tree import Tree
 
+from browser_cli.models import Tab
 from browser_cli.commands import rendering
 
 def test_shorten_uses_ellipsis():
@@ -32,3 +33,33 @@ def test_print_tree_uses_detected_width(monkeypatch):
   monkeypatch.setattr(rendering, "terminal_width", lambda console=None: 132)
   rendering.print_tree(Tree("Root"))
   assert widths == [132]
+
+def test_build_tabs_tree_groups_by_browser_window_and_group():
+  tabs = [
+    Tab(id=1, window_id=5, active=False, muted=False, title="Before", url="https://example.com/before", group_id=None, index=0, browser="work"),
+    Tab(id=2, window_id=5, active=False, muted=False, title="Inside", url="https://example.com/inside", group_id=9, index=1, browser="work"),
+  ]
+  groups = [{"id": 9, "windowId": 5, "browser": "work", "title": "Research", "color": "blue", "tabCount": 1, "collapsed": True}]
+  tree = rendering.build_tabs_tree(tabs, groups, console=Console(width=120), show_urls=True)
+  text = "\n".join(str(line) for line in tree.__rich_console__(Console(width=120), Console(width=120).options))
+  assert "work" in text
+  assert "Window 5" in text
+  assert "Research" in text
+  assert "collapsed" in text
+  assert "Inside" in text
+
+def test_build_windows_tree_keeps_multi_browser_windows_separate():
+  tabs = [
+    Tab(id=1, window_id=5, active=False, muted=False, title="Work Tab", url="https://example.com/work", index=0, browser="work"),
+    Tab(id=2, window_id=5, active=False, muted=False, title="Personal Tab", url="https://example.com/personal", index=0, browser="personal"),
+  ]
+  windows = [
+    {"id": 5, "alias": "main", "browser": "work", "tabCount": 1, "state": "normal"},
+    {"id": 5, "alias": "main", "browser": "personal", "tabCount": 1, "state": "normal"},
+  ]
+  tree = rendering.build_windows_tree(windows, tabs, console=Console(width=120))
+  text = "\n".join(str(line) for line in tree.__rich_console__(Console(width=120), Console(width=120).options))
+  assert "work: Window 5" in text
+  assert "personal: Window 5" in text
+  assert "Work Tab" in text
+  assert "Personal Tab" in text