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/storage.py

@@ -1,23 +1,22 @@
 import json
 import click
-from browser_cli.commands import _handle
+from browser_cli.commands import client_from_ctx, handle_errors, tab_option
 from rich.console import Console
 
 console = Console()
 
-
 @click.group("storage")
 def storage_group():
     """Read and write the page's localStorage / sessionStorage."""
 
-
 @storage_group.command("get")
 @click.argument("key", required=False)
 @click.option("--type", "store_type", type=click.Choice(["local", "session"]), default="local", show_default=True)
-@click.option("--tab", "tab_id", type=int, default=None, help="Tab ID (default: active tab)")
+@tab_option
+@handle_errors
 def storage_get(key, store_type, tab_id):
     """Get a localStorage KEY (or dump all keys if omitted)."""
-    result = _handle("storage.get", {"key": key, "type": store_type, "tabId": tab_id})
+    result = client_from_ctx().storage.get(key, type=store_type, tab_id=tab_id)
     if result is None:
         console.print("[dim]null[/dim]")
     elif isinstance(result, dict):
@@ -25,13 +24,13 @@ def storage_get(key, store_type, tab_id):
     else:
         console.print(str(result))
 
-
 @storage_group.command("set")
 @click.argument("key")
 @click.argument("value")
 @click.option("--type", "store_type", type=click.Choice(["local", "session"]), default="local", show_default=True)
-@click.option("--tab", "tab_id", type=int, default=None, help="Tab ID (default: active tab)")
+@tab_option
+@handle_errors
 def storage_set(key, value, store_type, tab_id):
     """Set localStorage KEY to VALUE."""
-    _handle("storage.set", {"key": key, "value": value, "type": store_type, "tabId": tab_id})
+    client_from_ctx().storage.set(key, value, type=store_type, tab_id=tab_id)
     console.print(f"[green]Set[/green] {store_type}[{key!r}] = {value!r}")