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/perf.py

@@ -1,7 +1,7 @@
 import click
 from rich.console import Console
 from rich.table import Table
-from browser_cli.commands import _handle
+from browser_cli.commands import client_from_ctx, handle_errors
 
 console = Console()
 
@@ -10,9 +10,10 @@ def perf_group():
     """Inspect and tune browser-cli performance behavior."""
 
 @perf_group.command("status")
+@handle_errors
 def perf_status():
     """Show performance profile, throttle and running jobs."""
-    result = _handle("perf.status") or {}
+    result = client_from_ctx().perf.status()
     console.print(f"Profile: [bold]{result.get('performanceProfile', 'auto')}[/bold]")
     console.print(f"Audible tabs: {'yes' if result.get('audible') else 'no'}")
     throttle = result.get("throttle") or {}
@@ -39,7 +40,8 @@ def perf_status():
 
 @perf_group.command("profile")
 @click.argument("profile", type=click.Choice(["auto", "normal", "gentle", "ultra"]))
+@handle_errors
 def perf_profile(profile):
     """Set global performance profile."""
-    result = _handle("perf.set_profile", {"profile": profile}) or {}
+    result = client_from_ctx().perf.set_profile(profile)
     console.print(f"[green]Performance profile set to {result.get('performanceProfile', profile)}[/green]")