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/sdk/base.py

@@ -0,0 +1,19 @@
+"""Base class for SDK command namespaces.
+
+Each namespace (``b.tabs``, ``b.dom``, ...) is a thin object bound to its
+:class:`~browser_cli.BrowserCLI` client. Namespaces hold no state of their own;
+they delegate to the client's shared infrastructure (``_cmd``, the multi-browser
+helpers, and the ``Tab``/``Group`` factories).
+"""
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from browser_cli import BrowserCLI
+
+class Namespace:
+    """A group of related SDK methods, bound to a BrowserCLI client."""
+
+    def __init__(self, client: "BrowserCLI"):
+        self._c = client