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/endpoints.py

@@ -0,0 +1,57 @@
+"""Remote endpoint string handling — parsing, normalization, display.
+
+Pure helpers (no sockets, no I/O) for turning user-facing ``host[:port]``
+strings into the canonical forms the rest of the client uses, and back into the
+short forms shown to humans. Re-exported from :mod:`browser_cli.client` for
+backward compatibility.
+"""
+from __future__ import annotations
+
+import re
+
+from browser_cli.errors import BrowserNotConnected
+
+_DEFAULT_REMOTE_PORT = 443
+
+def _looks_like_domain(host: str) -> bool:
+    """True if host looks like a domain name rather than an IP address or localhost."""
+    if host in {"localhost", "127.0.0.1", "::1"}:
+        return False
+    if re.match(r'^\d{1,3}(\.\d{1,3}){3}$', host):
+        return False
+    return '.' in host and any(c.isalpha() for c in host)
+
+def _normalize_endpoint(endpoint: str) -> str:
+    """Strip :443 from domain-like endpoints so they are stored without the default port."""
+    if not endpoint:
+        return endpoint
+    host, sep, port = endpoint.rpartition(":")
+    if sep and port == "443" and _looks_like_domain(host):
+        return host
+    return endpoint
+
+def _resolve_connect_endpoint(endpoint: str) -> str:
+    """Return host:port for TCP connection; domain without port defaults to :443."""
+    _, sep, _ = endpoint.rpartition(":")
+    if not sep:
+        if _looks_like_domain(endpoint):
+            return f"{endpoint}:{_DEFAULT_REMOTE_PORT}"
+        raise BrowserNotConnected(
+            f"Invalid remote endpoint '{endpoint}': expected host:port"
+        )
+    return endpoint
+
+def display_browser_name(profile_name: str, sock_path: str) -> str:
+    from pathlib import Path
+
+    if profile_name != "default":
+        return profile_name
+    return Path(sock_path).stem or profile_name
+
+def _remote_display_name(endpoint: str, profile_name: str, display_name: str) -> str:
+    host, sep, port = endpoint.rpartition(":")
+    if sep and (port == "8765" or (port == "443" and _looks_like_domain(host))):
+        display_endpoint = host
+    else:
+        display_endpoint = endpoint  # normalized domain (no port) or non-default port
+    return f"{display_endpoint}:{display_name or profile_name}"