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/navigate.py

@@ -1,23 +1,22 @@
 import click
-from browser_cli.commands import _handle
+from browser_cli.commands import client_from_ctx, handle_errors, tab_option
 from rich.console import Console
 
 console = Console()
 
-
 @click.group("nav")
 def nav_group():
     """Navigate — open URLs, reload, go back/forward, focus tabs."""
 
-
 @nav_group.command("open")
 @click.argument("url")
 @click.option("--bg", is_flag=True, help="Open in background (no focus)")
 @click.option("--window", "window_name", default=None, help="Open in named window")
 @click.option("--group", "group_name", default=None, help="Open directly into a tab group (name or ID)")
+@handle_errors
 def cmd_open(url, bg, window_name, group_name):
     """Open URL in a new tab."""
-    _handle("navigate.open", {"url": url, "background": bg, "window": window_name, "group": group_name})
+    client_from_ctx().nav.open(url, background=bg, window=window_name, group=group_name)
     suffix = ""
     if group_name:
         suffix = f" in group '{group_name}'"
@@ -25,71 +24,67 @@ def cmd_open(url, bg, window_name, group_name):
         suffix = f" in window '{window_name}'"
     console.print(f"[green]Opened:[/green] {url}{suffix}")
 
-
 @nav_group.command("reload")
 @click.argument("tab_id", type=int, required=False)
+@handle_errors
 def cmd_reload(tab_id):
     """Reload the active (or specified) tab."""
-    _handle("navigate.reload", {"tabId": tab_id})
+    client_from_ctx().nav.reload(tab_id)
     console.print("[green]Reloaded[/green]")
 
-
 @nav_group.command("hard-reload")
 @click.argument("tab_id", type=int, required=False)
+@handle_errors
 def cmd_hard_reload(tab_id):
     """Hard reload (bypass cache) the active (or specified) tab."""
-    _handle("navigate.hard_reload", {"tabId": tab_id})
+    client_from_ctx().nav.hard_reload(tab_id)
     console.print("[green]Hard reloaded[/green]")
 
-
 @nav_group.command("back")
 @click.argument("tab_id", type=int, required=False)
+@handle_errors
 def cmd_back(tab_id):
     """Navigate back in the active (or specified) tab."""
-    _handle("navigate.back", {"tabId": tab_id})
+    client_from_ctx().nav.back(tab_id)
     console.print("[green]Navigated back[/green]")
 
-
 @nav_group.command("forward")
 @click.argument("tab_id", type=int, required=False)
+@handle_errors
 def cmd_forward(tab_id):
     """Navigate forward in the active (or specified) tab."""
-    _handle("navigate.forward", {"tabId": tab_id})
+    client_from_ctx().nav.forward(tab_id)
     console.print("[green]Navigated forward[/green]")
 
-
 @nav_group.command("focus")
 @click.argument("pattern")
+@handle_errors
 def cmd_focus(pattern):
     """Jump to a tab by URL pattern or tab ID."""
-    result = _handle("navigate.focus", {"pattern": pattern})
+    result = client_from_ctx().nav.focus(pattern)
     if result:
         console.print(f"[green]Focused:[/green] {result.get('url', result)}")
     else:
         console.print(f"[yellow]No tab found matching:[/yellow] {pattern}")
 
-
 @nav_group.command("open-wait")
 @click.argument("url")
 @click.option("--timeout", type=float, default=30.0, show_default=True, help="Max seconds to wait for load")
 @click.option("--bg", is_flag=True, help="Open in background (no focus)")
 @click.option("--window", "window_name", default=None, help="Open in named window")
 @click.option("--group", "group_name", default=None, help="Open in tab group")
+@handle_errors
 def cmd_open_wait(url, timeout, bg, window_name, group_name):
     """Open URL in a new tab and wait until fully loaded."""
-    result = _handle("navigate.open_wait", {
-        "url": url, "timeout": int(timeout * 1000),
-        "background": bg, "window": window_name, "group": group_name,
-    })
-    title = result.get("title", "") if isinstance(result, dict) else ""
-    console.print(f"[green]Loaded:[/green] {url}" + (f" — {title}" if title else ""))
-
+    tab = client_from_ctx().nav.open_wait(url, timeout=timeout, background=bg, window=window_name, group=group_name)
+    console.print(f"[green]Loaded:[/green] {url}" + (f" — {tab.title}" if tab.title else ""))
 
 @nav_group.command("wait")
-@click.option("--tab", "tab_id", type=int, default=None, help="Tab ID (default: active tab)")
+@tab_option
 @click.option("--timeout", type=float, default=30.0, show_default=True, help="Max seconds to wait")
 @click.option("--ready-state", type=click.Choice(["complete", "interactive"]), default="complete", show_default=True, help="Target ready state")
+@handle_errors
 def cmd_wait(tab_id, timeout, ready_state):
     """Wait until tab finishes loading."""
-    result = _handle("navigate.wait", {"tabId": tab_id, "timeout": int(timeout * 1000), "readyState": ready_state})
-    console.print(f"[green]Ready:[/green] {result.get('url', '')} — {result.get('title', '')}")
+    tab = client_from_ctx().tabs.wait_for_load(tab_id, timeout=timeout, ready_state=ready_state)
+    console.print(f"[green]Ready:[/green] {tab.url} — {tab.title}")