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/commands/script.py

@@ -1,7 +1,9 @@
 from __future__ import annotations
 
+import importlib
 import json
 from pathlib import Path
+from typing import Any, cast
 
 import click
 from rich.console import Console
@@ -12,25 +14,25 @@ from browser_cli.commands import client_from_ctx, handle_errors
 console = Console()
 
 def _load_steps(path: Path):
-    text = path.read_text(encoding="utf-8")
-    if path.suffix.lower() in {".yaml", ".yml"}:
-        try:
-            import yaml  # type: ignore
-        except Exception as exc:
-            raise click.ClickException("YAML scripts require PyYAML; use JSON or install PyYAML") from exc
-        return yaml.safe_load(text)
-    return json.loads(text)
+  text = path.read_text(encoding="utf-8")
+  if path.suffix.lower() in {".yaml", ".yml"}:
+    try:
+      yaml = cast(Any, importlib.import_module("yaml"))
+    except Exception as exc:
+      raise click.ClickException("YAML scripts require PyYAML; use JSON or install PyYAML") from exc
+    return yaml.safe_load(text)
+  return json.loads(text)
 
 def _parse_step(step):
-    if isinstance(step, str):
-        return step, {}
-    if isinstance(step, dict):
-        if "command" in step:
-            return step["command"], step.get("args") or {}
-        if len(step) == 1:
-            command, args = next(iter(step.items()))
-            return command, args or {}
-    raise click.ClickException(f"Invalid script step: {step!r}")
+  if isinstance(step, str):
+    return step, {}
+  if isinstance(step, dict):
+    if "command" in step:
+      return step["command"], step.get("args") or {}
+    if len(step) == 1:
+      command, args = next(iter(step.items()))
+      return command, args or {}
+  raise click.ClickException(f"Invalid script step: {step!r}")
 
 @click.command("script")
 @click.argument("file", type=click.Path(exists=True, dir_okay=False, path_type=Path))
@@ -41,28 +43,28 @@ def _parse_step(step):
 @click.option("--allow-dangerous", is_flag=True, help="Allow high-risk commands such as dom.eval, storage.*, screenshots")
 @handle_errors
 def cmd_script(file: Path, json_output: bool, continue_on_error: bool, allow_read_page: bool, allow_control: bool, allow_dangerous: bool):
-    """Run a JSON/YAML batch script of browser-cli wire commands."""
-    steps = _load_steps(file)
-    if not isinstance(steps, list):
-        raise click.ClickException("Script root must be a list")
-    client = client_from_ctx()
-    policy = CommandPolicy(allow_read_page=allow_read_page, allow_control=allow_control, allow_dangerous=allow_dangerous)
-    results = []
-    for index, step in enumerate(steps, start=1):
-        command, args = _parse_step(step)
-        try:
-            assert_command_allowed(command, policy)
-            result = client.command(command, args)
-            results.append({"index": index, "command": command, "ok": True, "result": result})
-            if not json_output:
-                console.print(f"[green]✓[/green] {index}: {command}")
-        except Exception as exc:
-            results.append({"index": index, "command": command, "ok": False, "error": str(exc)})
-            if not continue_on_error:
-                if json_output:
-                    click.echo(json.dumps(results, indent=2, default=str))
-                raise
-            if not json_output:
-                console.print(f"[red]✗[/red] {index}: {command}: {exc}")
-    if json_output:
-        click.echo(json.dumps(results, indent=2, default=str))
+  """Run a JSON/YAML batch script of browser-cli wire commands."""
+  steps = _load_steps(file)
+  if not isinstance(steps, list):
+    raise click.ClickException("Script root must be a list")
+  client = client_from_ctx()
+  policy = CommandPolicy(allow_read_page=allow_read_page, allow_control=allow_control, allow_dangerous=allow_dangerous)
+  results = []
+  for index, step in enumerate(steps, start=1):
+    command, args = _parse_step(step)
+    try:
+      assert_command_allowed(command, policy)
+      result = client.command(command, args)
+      results.append({"index": index, "command": command, "ok": True, "result": result})
+      if not json_output:
+        console.print(f"[green]✓[/green] {index}: {command}")
+    except Exception as exc:
+      results.append({"index": index, "command": command, "ok": False, "error": str(exc)})
+      if not continue_on_error:
+        if json_output:
+          click.echo(json.dumps(results, indent=2, default=str))
+        raise
+      if not json_output:
+        console.print(f"[red]✗[/red] {index}: {command}: {exc}")
+  if json_output:
+    click.echo(json.dumps(results, indent=2, default=str))