refactor: collapse strix/sandbox into strix/runtime; in-sandbox Caido bootstrap

The split between ``strix/sandbox/`` and ``strix/runtime/`` was
artificial — both were managing the same backend. ``strix/sandbox/``
also collided uncomfortably with the SDK's ``agents.sandbox.*``
namespace. ``runtime/`` (which matches ``STRIX_RUNTIME_BACKEND``) is
the canonical home for everything Docker / Daytona / K8s lifecycle.

While merging, also rip out two pieces of Docker-specific coupling:

- ``caido_bootstrap`` was POSTing ``loginAsGuest`` from the host via
  ``aiohttp`` to ``http://127.0.0.1:{forwarded_port}``. That assumed
  Docker port forwarding; Daytona / K8s expose ports differently.
  Now we ``session.exec`` curl from *inside* the container — the
  SDK's runtime-agnostic exec primitive — so any backend works as
  long as it implements ``exec``. The host-side Caido ``Client``
  still uses the runtime's exposed-port URL for post-bootstrap calls,
  but that goes through the SDK's own ``resolve_exposed_port``
  abstraction (also runtime-agnostic).

- The bootstrap retry loop now doubles as the readiness probe, so
  ``healthcheck.wait_for_tcp_ready`` (and the entire
  ``healthcheck.py`` module) goes away.

Drive-by simplification: drop ``caido_host_port`` plumbing entirely.
It was only piped through ``make_agent_context`` → child contexts
without ever being read; only ``caido_client`` is consumed.

Drops ``aiohttp`` runtime dep (it stays only as a transitive of the
Caido SDK).

strix/interface/cli.py

@@ -15,7 +15,7 @@ from rich.text import Text
 
 from strix.config import Config
 from strix.entry import run_strix_scan
-from strix.sandbox import session_manager
+from strix.runtime import session_manager
 from strix.telemetry.tracer import Tracer, set_global_tracer
 
 from .utils import (

strix/interface/tui.py

@@ -37,7 +37,7 @@ from strix.interface.tool_components.agent_message_renderer import AgentMessageR
 from strix.interface.tool_components.registry import get_tool_renderer
 from strix.interface.tool_components.user_message_renderer import UserMessageRenderer
 from strix.interface.utils import build_tui_stats_text
-from strix.sandbox import session_manager
+from strix.runtime import session_manager
 from strix.telemetry.tracer import Tracer, set_global_tracer