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()).

tests/test_tabs.py

@@ -1,4 +1,5 @@
 """Tests for tabs.* commands."""
+import time
 import pytest
 
 def test_tabs_list(browser):
@@ -72,6 +73,16 @@ def test_tabs_dedupe(browser):
     id1, id2 = r1["id"], r2["id"]
 
     try:
+        # Dedupe keys on tab.url and intentionally skips tabs that haven't
+        # resolved a URL yet, so wait for both tabs to finish loading first.
+        for _ in range(30):
+            tabs = {t["id"]: t for t in browser("tabs.list")}
+            if all(tabs.get(tid, {}).get("url", "").startswith("http") for tid in (id1, id2)):
+                break
+            time.sleep(0.1)
+        else:
+            pytest.skip("Duplicate test tabs did not finish loading")
+
         result = browser("tabs.dedupe")
         assert isinstance(result, dict)
         assert result.get("closed", 0) >= 0