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/models.py

@@ -4,11 +4,11 @@ Typed dataclasses returned by the BrowserCLI Python API.
 Each object is bound to a BrowserCLI instance so you can call actions
 directly on it:
 
-    tabs = b.tabs_list()
+    tabs = b.tabs.list()
     tabs[0].close()
     tabs[0].move(forward=True)
 
-    groups = b.group_list()
+    groups = b.groups.list()
     groups[0].tabs()
     groups[0].add_tab("https://example.com")
 """
@@ -21,6 +21,14 @@ if TYPE_CHECKING:
     from browser_cli import BrowserCLI
 
 
+# ── BrowserCounts ───────────────────────────────────────────────────────────
+
+@dataclass(frozen=True)
+class BrowserCounts:
+    """Aggregated per-browser counts returned in implicit multi-browser mode."""
+    total: int
+    by_browser: dict[str, int]
+
 # ── Tab ───────────────────────────────────────────────────────────────────────
 
 @dataclass
@@ -97,7 +105,7 @@ class Tab:
 
     def screenshot(self, *, format: str = "png", quality: int | None = None) -> str:
         """Capture this tab's visible area. Returns a base64 data URL."""
-        return self._b().tabs_screenshot(self.id, format=format, quality=quality)
+        return self._b().tabs.screenshot(self.id, format=format, quality=quality)
 
     def pin(self) -> None:
         """Pin this tab."""
@@ -109,19 +117,19 @@ class Tab:
 
     def refresh(self) -> Tab:
         """Return a fresh snapshot of this tab."""
-        return self._b().tabs_status(self.id)
+        return self._b().tabs.status(self.id)
 
     def wait_for_load(self, *, timeout: float = 30.0, ready_state: str = "complete") -> Tab:
         """Wait until this tab reaches the requested readyState."""
-        return self._b().wait_for_load(self.id, timeout=timeout, ready_state=ready_state)
+        return self._b().tabs.wait_for_load(self.id, timeout=timeout, ready_state=ready_state)
 
     def watch_url(self, pattern: str, *, timeout: float = 30.0) -> Tab:
         """Wait until this tab's URL matches regex *pattern*."""
-        return self._b().tabs_watch_url(pattern, tab_id=self.id, timeout=timeout)
+        return self._b().tabs.watch_url(pattern, tab_id=self.id, timeout=timeout)
 
     def open(self, url: str, *, background: bool = False) -> None:
         """Navigate this tab to *url* in place."""
-        self._b().navigate_tab(self.id, url)
+        self._b().nav.to(self.id, url)
 
 
 # ── Group ─────────────────────────────────────────────────────────────────────
@@ -148,7 +156,7 @@ class Group:
 
     def tabs(self) -> list[Tab]:
         """Return all tabs inside this group."""
-        return self._b().group_tabs(self.id)
+        return self._b().groups.tabs(self.id)
 
     def move(self, *, forward: bool = False, backward: bool = False) -> None:
         """Move this group forward or backward among groups."""
@@ -160,4 +168,4 @@ class Group:
 
     def add_tab(self, url: str | None = None) -> int | None:
         """Open a new tab inside this group. Returns the new tab ID."""
-        return self._b().group_add_tab(self.id, url)
+        return self._b().groups.add_tab(self.id, url)