refactor(api): namespaced SDK + dedicated transport layer

Restructure the Python API and internals around composable namespaces and
a standalone transport/endpoint layer. Bump to 0.12.0.

Python API:
- Replace flat methods (b.tabs_list(), b.group_list()) with namespaces:
  b.nav, b.tabs, b.groups, b.windows, b.dom, b.extract, b.page, b.storage,
  b.cookies, b.session, b.perf, b.extension.
- Shrink browser_cli/__init__.py to a thin composition root; move all
  behaviour into browser_cli/sdk/ (one module per namespace + factories,
  base, routing).

Internals:
- Add browser_cli/transport.py and remote_transport.py to isolate IPC from
  command logic; client.py now delegates instead of owning transport.
- Add browser_cli/endpoints.py for endpoint resolution and
  browser_cli/errors.py for shared error types.
- Extract markdown rendering into browser_cli/markdown.py (out of extract).
- Add USER_AGENT to version_manager.

Tooling & tests:
- Add justfile with common dev tasks.
- Update CLI commands and demo to the namespaced API.
- Rework tests for the new layout; add test_transport.py and
  test_refactor_boundaries.py to lock in module boundaries.

BREAKING CHANGE: flat API methods are removed in favour of namespaces
(e.g. b.tabs_list() -> b.tabs.list(), b.group_list() -> b.groups.list()).

browser_cli/commands/search.py

@@ -1,6 +1,5 @@
 import click
-from urllib.parse import quote_plus
-from browser_cli.commands import _handle
+from browser_cli.commands import client_from_ctx, handle_errors
 from rich.console import Console
 
 console = Console()
@@ -61,23 +60,21 @@ _SUBCOMMANDS = [
 def search_group():
     """Search the web — open a query in a search engine."""
 
-
 def _build_command(engine_key: str, help_text: str) -> click.Command:
     @click.command(engine_key, help=help_text)
     @click.argument("query", nargs=-1, required=True)
     @click.option("--bg", is_flag=True, help="Open in background (no focus)")
     @click.option("--window", "window", default=None, help="Open in named window")
     @click.option("--group", "group", default=None, help="Open in tab group (name or ID)")
+    @handle_errors
     def _cmd(query, bg, window, group):
         terms = " ".join(query)
-        url = ENGINES[engine_key].format(query=quote_plus(terms))
-        _handle("navigate.open", {"url": url, "background": bg, "window": window, "group": group})
+        client_from_ctx().nav.search(engine_key, terms, background=bg, window=window, group=group)
         suffix = f" in group '{group}'" if group else (f" in window '{window}'" if window else "")
         display = _DISPLAY_NAMES.get(engine_key, engine_key.capitalize())
         console.print(f"[green]Searching[/green] [cyan]{display}[/cyan]: {terms}{suffix}")
 
     return _cmd
 
-
 for _name, _help in _SUBCOMMANDS:
     search_group.add_command(_build_command(_name, _help))