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/session.py

@@ -1,5 +1,5 @@
 import click
-from browser_cli.commands import _handle, _handle_multi, _multi_browser_targets
+from browser_cli.commands import client_from_ctx, gentle_mode_option, handle_errors
 from rich.console import Console
 
 console = Console()
@@ -10,41 +10,47 @@ def session_group():
 
 @session_group.command("save")
 @click.argument("name")
+@handle_errors
 def session_save(name):
     """Save all current tabs as session NAME."""
-    result = _handle("session.save", {"name": name})
+    result = client_from_ctx().session.save(name)
     count = result.get("tabs", 0) if isinstance(result, dict) else 0
     console.print(f"[green]Session '{name}' saved[/green] ({count} tabs)")
 
 @session_group.command("load")
 @click.argument("name")
-@click.option("--gentle-mode", type=click.Choice(["auto", "normal", "gentle", "ultra"]), default="auto", show_default=True, help="Throttle mode for large restores.")
+@gentle_mode_option("Throttle mode for large restores.")
 @click.option("--discard-background-tabs", is_flag=True, help="Discard restored background tabs after opening to reduce load.")
 @click.option("--lazy", is_flag=True, help="Create lightweight placeholder tabs after --eager-tabs; placeholders load when selected.")
 @click.option("--eager-tabs", type=int, default=10, show_default=True, help="Number of real tabs to open before lazy placeholders.")
 @click.option("--background", "background_job", is_flag=True, help="Start restore as a background job and return immediately.")
+@handle_errors
 def session_load(name, gentle_mode, discard_background_tabs, lazy, eager_tabs, background_job):
     """Restore session NAME (opens all saved tabs)."""
-    result = _handle("session.load", {
-        "name": name,
-        "gentleMode": gentle_mode,
-        "discardBackgroundTabs": discard_background_tabs,
-        "lazy": lazy,
-        "eagerTabs": eager_tabs,
-        "__background": background_job,
-    })
-    if background_job and isinstance(result, dict) and result.get("jobId"):
-        console.print(f"[green]Session restore started[/green] job={result['jobId']}")
-        return
+    b = client_from_ctx()
+    if background_job:
+        result = b.session.load_background(
+            name, gentle_mode=gentle_mode, discard_background_tabs=discard_background_tabs,
+            lazy=lazy, eager_tabs=eager_tabs,
+        )
+        if isinstance(result, dict) and result.get("jobId"):
+            console.print(f"[green]Session restore started[/green] job={result['jobId']}")
+            return
+    else:
+        result = b.session.load(
+            name, gentle_mode=gentle_mode, discard_background_tabs=discard_background_tabs,
+            lazy=lazy, eager_tabs=eager_tabs,
+        )
     count = result.get("tabs", 0) if isinstance(result, dict) else 0
     console.print(f"[green]Session '{name}' loaded[/green] ({count} tabs opened)")
 
 @session_group.command("diff")
 @click.argument("name_a")
 @click.argument("name_b")
+@handle_errors
 def session_diff(name_a, name_b):
     """Show tabs added/removed between two saved sessions."""
-    diff = _handle("session.diff", {"nameA": name_a, "nameB": name_b})
+    diff = client_from_ctx().session.diff(name_a, name_b)
     if not diff:
         console.print("[yellow]No diff data returned[/yellow]")
         return
@@ -66,26 +72,16 @@ def session_diff(name_a, name_b):
         console.print("[green]Sessions are identical[/green]")
 
 @session_group.command("list")
+@handle_errors
 def session_list():
     """List all saved sessions."""
+    from datetime import datetime
     from rich.table import Table
-    targets = _multi_browser_targets()
-    show_browser = bool(targets)
-    if targets:
-        sessions = []
-        for target in targets:
-            result = _handle_multi("session.list", profile=target.profile, remote=target.remote)
-            if result is None:
-                continue
-            sessions.extend({**session, "browser": target.display_name} for session in result)
-        if not sessions:
-            console.print("[red]Error:[/red] Cannot resolve a browser socket automatically.")
-            raise SystemExit(1)
-    else:
-        sessions = _handle("session.list")
+    sessions = client_from_ctx().session.list()
     if not sessions:
         console.print("[yellow]No saved sessions[/yellow]")
         return
+    show_browser = any("browser" in s for s in sessions)
     table = Table(show_header=True, header_style="bold cyan")
     if show_browser:
         table.add_column("Browser")
@@ -93,7 +89,6 @@ def session_list():
     table.add_column("Tabs", width=6)
     table.add_column("Saved at")
     for s in sessions:
-        from datetime import datetime
         saved = datetime.fromtimestamp(s["savedAt"] / 1000).strftime("%Y-%m-%d %H:%M") if s.get("savedAt") else ""
         row = [s.get("browser", "")] if show_browser else []
         row.extend([s["name"], str(s["tabs"]), saved])
@@ -102,16 +97,18 @@ def session_list():
 
 @session_group.command("remove")
 @click.argument("name")
+@handle_errors
 def session_remove(name):
     """Delete a saved session."""
-    _handle("session.remove", {"name": name})
+    client_from_ctx().session.remove(name)
     console.print(f"[green]Session '{name}' removed[/green]")
 
 @session_group.command("job-status")
 @click.argument("job_id")
+@handle_errors
 def session_job_status(job_id):
     """Show status for a background session job."""
-    result = _handle("jobs.status", {"jobId": job_id}) or {}
+    result = client_from_ctx().perf.job_status(job_id)
     status = result.get("status", "unknown")
     console.print(f"[bold]{job_id}[/bold]: {status}")
     if result.get("error"):
@@ -121,15 +118,17 @@ def session_job_status(job_id):
 
 @session_group.command("job-cancel")
 @click.argument("job_id")
+@handle_errors
 def session_job_cancel(job_id):
     """Cancel a running background job."""
-    _handle("jobs.cancel", {"jobId": job_id})
+    client_from_ctx().perf.job_cancel(job_id)
     console.print(f"[green]Cancel requested for {job_id}[/green]")
 
 @session_group.command("auto-save")
 @click.argument("state", type=click.Choice(["on", "off"]))
+@handle_errors
 def session_auto_save(state):
     """Enable or disable automatic session saving."""
     enabled = state == "on"
-    _handle("session.auto_save", {"enabled": enabled})
+    client_from_ctx().session.auto_save(enabled)
     console.print(f"[green]Auto-save {state}[/green]")