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_extension_error_page_handling.py

@@ -111,7 +111,6 @@ def test_large_extension_operations_yield_between_batches():
     assert "perf.set_profile" in perf
     assert "__background" in connection
 
-
 def test_session_autosave_is_debounced_and_non_overlapping():
     # The autosave lifecycle moved out of session.ts into a dedicated
     # AutoSaveManager (autosave.ts) during the structure refactor; the shared
@@ -137,31 +136,34 @@ def test_session_autosave_is_debounced_and_non_overlapping():
     assert "setTimeout(() => this.runAutoSave(), delayMs)" in autosave
     assert "clearTimeout(this.autoSaveTimer)" in autosave
 
-
 def test_cli_and_sdk_expose_gentle_restore_controls():
     session_cli = (ROOT / "browser_cli" / "commands" / "session.py").read_text()
     tabs_cli = (ROOT / "browser_cli" / "commands" / "tabs.py").read_text()
     groups_cli = (ROOT / "browser_cli" / "commands" / "groups.py").read_text()
-    sdk = (ROOT / "browser_cli" / "__init__.py").read_text()
+    commands_init = (ROOT / "browser_cli" / "commands" / "__init__.py").read_text()
+    sdk_session = (ROOT / "browser_cli" / "sdk" / "session.py").read_text()
+    sdk_perf = (ROOT / "browser_cli" / "sdk" / "perf.py").read_text()
 
-    assert "--gentle-mode" in session_cli
+    # The --gentle-mode flag is defined once via the shared gentle_mode_option helper.
+    assert '"--gentle-mode"' in commands_init
+    assert "gentle_mode_option" in session_cli
     assert "--discard-background-tabs" in session_cli
     assert "--background" in session_cli
     assert "--lazy" in session_cli
     assert "--eager-tabs" in session_cli
     assert "job-status" in session_cli
     assert "job-cancel" in session_cli
-    assert "discardBackgroundTabs" in session_cli
-    assert "--gentle-mode" in tabs_cli
-    assert "gentleMode" in tabs_cli
-    assert "--gentle-mode" in groups_cli
-    assert "discard_background_tabs" in sdk
-    assert "discardBackgroundTabs" in sdk
-    assert "session_load_background" in sdk
-    assert "job_status" in sdk
-    assert "job_cancel" in sdk
-    assert "perf_status" in sdk
-    assert "set_performance_profile" in sdk
+    assert "discard_background_tabs" in session_cli
+    assert "gentle_mode_option" in tabs_cli
+    assert "gentle_mode" in tabs_cli
+    assert "gentle_mode_option" in groups_cli
+    assert "discard_background_tabs" in sdk_session
+    assert "discardBackgroundTabs" in sdk_session
+    assert "def load_background" in sdk_session
+    assert "def job_status" in sdk_perf
+    assert "def job_cancel" in sdk_perf
+    assert "def status" in sdk_perf
+    assert "def set_profile" in sdk_perf
 
     perf_cli = (ROOT / "browser_cli" / "commands" / "perf.py").read_text()
     root_cli = (ROOT / "browser_cli" / "cli.py").read_text()