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/windows.py

@@ -0,0 +1,24 @@
+"""Windows namespace: ``b.windows.*``."""
+from __future__ import annotations
+
+from browser_cli.sdk.base import Namespace
+
+class WindowsNS(Namespace):
+    """List, open, close, and rename browser windows."""
+
+    def list(self) -> list[dict]:
+        """Return browser windows.
+
+        In implicit multi-browser mode each window dict includes a ``browser`` key.
+        """
+        return self._c._multi_list("windows.list", {}, self._c._tag_browser)
+
+    def open(self, url: str | None = None) -> dict:
+        """Open a new browser window, optionally on a URL."""
+        return self._c._cmd("windows.open", {"url": url}) or {}
+
+    def close(self, window_id: int) -> None:
+        self._c._cmd("windows.close", {"windowId": window_id})
+
+    def rename(self, window_id: int, name: str) -> None:
+        self._c._cmd("windows.rename", {"windowId": window_id, "name": name})