refactor: reorganize client transport and extension internals

- Split client, native, remote, serve, markdown, and SDK internals into focused packages with direct imports.
- Move local and remote transport framing/protocol helpers behind clearer module boundaries.
- Break up the extension injected DOM logic into a separate content dispatch bundle and dedicated content modules.
- Add explicit client handling for passive remote discovery without noisy PQ warnings.
- Keep behavior covered with updated unit, integration, and extension tests.

browser_cli/commands/extract.py

@@ -2,9 +2,6 @@ import json
 
 import click
 from browser_cli.commands import client_from_ctx, handle_errors
-# Re-exported for backward compatibility: the HTML→Markdown engine now lives in
-# browser_cli.markdown and is applied by the SDK (ExtractNS.markdown).
-from browser_cli.markdown import _clean_markdown_output, _convert_html_to_markdown  # noqa: F401
 from rich.console import Console
 from rich.table import Table