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/windows.py

@@ -1,11 +1,10 @@
 import click
-from browser_cli.commands import _handle, _handle_multi, _multi_browser_targets
+from browser_cli.commands import client_from_ctx, handle_errors
 from rich.console import Console
 from rich.table import Table
 
 console = Console()
 
-
 def _print_windows(windows: list[dict], *, show_browser: bool = False) -> None:
     if not windows:
         console.print("[yellow]No windows found[/yellow]")
@@ -28,53 +27,39 @@ def _print_windows(windows: list[dict], *, show_browser: bool = False) -> None:
         table.add_row(*[value for value in row if value is not None])
     console.print(table)
 
-
 @click.group("windows")
 def windows_group():
     """Manage browser windows."""
 
-
 @windows_group.command("list")
+@handle_errors
 def windows_list():
     """List all browser windows."""
-    targets = _multi_browser_targets()
-    if targets:
-        windows = []
-        for target in targets:
-            result = _handle_multi("windows.list", profile=target.profile, remote=target.remote)
-            if result is None:
-                continue
-            windows.extend({**window, "browser": target.display_name} for window in result)
-        if not windows:
-            console.print("[red]Error:[/red] Cannot resolve a browser socket automatically.")
-            raise SystemExit(1)
-        _print_windows(windows, show_browser=True)
-        return
-    windows = _handle("windows.list")
-    _print_windows(windows or [])
-
+    windows = client_from_ctx().windows.list()
+    _print_windows(windows, show_browser=any("browser" in w for w in windows))
 
 @windows_group.command("rename")
 @click.argument("window_id", type=int)
 @click.argument("name")
+@handle_errors
 def windows_rename(window_id, name):
     """Give a window a local alias NAME (stored in native host)."""
-    _handle("windows.rename", {"windowId": window_id, "name": name})
+    client_from_ctx().windows.rename(window_id, name)
     console.print(f"[green]Window {window_id} aliased as '{name}'[/green]")
 
-
 @windows_group.command("close")
 @click.argument("window_id", type=int)
+@handle_errors
 def windows_close(window_id):
     """Close a browser window."""
-    _handle("windows.close", {"windowId": window_id})
+    client_from_ctx().windows.close(window_id)
     console.print(f"[green]Window {window_id} closed[/green]")
 
-
 @windows_group.command("open")
 @click.argument("url", required=False)
+@handle_errors
 def windows_open(url):
     """Open a new browser window."""
-    result = _handle("windows.open", {"url": url})
+    result = client_from_ctx().windows.open(url)
     wid = result.get("id") if isinstance(result, dict) else result
     console.print(f"[green]Opened new window[/green] (id: {wid})" + (f" with {url}" if url else ""))