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).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

uv.lock

@@ -2038,7 +2038,6 @@ name = "strix-agent"
 version = "0.8.3"
 source = { editable = "." }
 dependencies = [
-    { name = "aiohttp" },
     { name = "caido-sdk-client" },
     { name = "cvss" },
     { name = "docker" },
@@ -2061,7 +2060,6 @@ dev = [
 
 [package.metadata]
 requires-dist = [
-    { name = "aiohttp", specifier = ">=3.10.0" },
     { name = "caido-sdk-client", specifier = ">=0.2.0" },
     { name = "cvss", specifier = ">=3.2" },
     { name = "docker", specifier = ">=7.1.0" },