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/cookies.py

@@ -1,23 +1,22 @@
 import click
-from browser_cli.commands import _handle
+from browser_cli.commands import client_from_ctx, handle_errors
 from rich.console import Console
 from rich.table import Table
 
 console = Console()
 
-
 @click.group("cookies")
 def cookies_group():
     """Manage browser cookies."""
 
-
 @cookies_group.command("list")
 @click.option("--url", default=None, help="Filter by URL")
 @click.option("--domain", default=None, help="Filter by domain")
 @click.option("--name", default=None, help="Filter by cookie name")
+@handle_errors
 def cookies_list(url, domain, name):
     """List cookies, optionally filtered by URL, domain, or name."""
-    cookies = _handle("cookies.list", {"url": url, "domain": domain, "name": name}) or []
+    cookies = client_from_ctx().cookies.list(url=url, domain=domain, name=name)
     if not cookies:
         console.print("[yellow]No cookies found[/yellow]")
         return
@@ -39,19 +38,18 @@ def cookies_list(url, domain, name):
         )
     console.print(table)
 
-
 @cookies_group.command("get")
 @click.argument("url")
 @click.argument("name")
+@handle_errors
 def cookies_get(url, name):
     """Get the value of a single cookie by URL and NAME."""
-    cookie = _handle("cookies.get", {"url": url, "name": name})
+    cookie = client_from_ctx().cookies.get(url, name)
     if cookie is None:
         console.print(f"[yellow]Cookie '{name}' not found for {url}[/yellow]")
         raise SystemExit(1)
     console.print(cookie.get("value", ""))
 
-
 @cookies_group.command("set")
 @click.argument("url")
 @click.argument("name")
@@ -62,14 +60,13 @@ def cookies_get(url, name):
 @click.option("--http-only", "http_only", is_flag=True)
 @click.option("--expires", "expiration_date", type=float, default=None, help="Unix timestamp")
 @click.option("--same-site", type=click.Choice(["no_restriction", "lax", "strict"]), default=None)
+@handle_errors
 def cookies_set(url, name, value, domain, path, secure, http_only, expiration_date, same_site):
     """Set a cookie on URL."""
-    _handle("cookies.set", {
-        "url": url, "name": name, "value": value,
-        "domain": domain, "path": path,
-        "secure": secure or None,
-        "httpOnly": http_only or None,
-        "expirationDate": expiration_date,
-        "sameSite": same_site,
-    })
+    client_from_ctx().cookies.set(
+        url, name, value,
+        domain=domain, path=path,
+        secure=secure or None, http_only=http_only or None,
+        expiration_date=expiration_date, same_site=same_site,
+    )
     console.print(f"[green]Set cookie:[/green] {name}={value!r} on {url}")