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/extension.py

@@ -1,7 +1,6 @@
-import time
 import click
 from rich.console import Console
-from browser_cli.commands import _handle
+from browser_cli.commands import client_from_ctx, handle_errors
 
 console = Console()
 
@@ -10,6 +9,7 @@ def extension_group():
     """Manage the browser-cli browser extension."""
 
 @extension_group.command("reload")
+@handle_errors
 def extension_reload():
     """Reload the browser-cli extension service worker.
 
@@ -17,5 +17,5 @@ def extension_reload():
     The command returns immediately; the extension restarts ~200 ms later.
     Re-connects automatically via the keepalive alarm within ~25 seconds.
     """
-    _handle("extension.reload")
+    client_from_ctx().extension.reload()
     console.print("[green]Extension reloading…[/green] reconnects automatically")