ci: backend-agnostic integration guards + per-backend preflight

Integration tests now select their backend from BOT_BOTTLE_BACKEND and
skip on the capability that backend actually needs, instead of gating
every backend on unrelated Docker availability.

Task 1 — backend-agnostic guards (tests/_backend.py):
- Capability probes: docker_capability() (reachable daemon) and
  firecracker_capability() (accessible /dev/kvm + firecracker on PATH,
  Docker-independent). backend_capability()/selected_backend() resolve
  the target from BOT_BOTTLE_BACKEND (default docker).
- skip_unless_selected_backend_available() for backend-agnostic tests
  (test_sandbox_escape) — runs through whichever backend is selected and
  checks that backend's real capability.
- skip_unless_backend("docker") for Docker-implementation tests
  (DockerBroker, DockerGateway, backend.docker.*) — they no-op under a
  non-Docker run rather than testing internals that run doesn't target.
- Retires tests/_docker.py; the KVM job no longer needs SKIP_DOCKER_TESTS
  to steer Docker-only classes.

Task 2 — explicit per-backend skip visibility:
- tests/backend_preflight.py prints a clear PASS/FAIL capability line and
  exits non-zero when the selected backend is missing.
- Both integration jobs run it as a preflight, so absent infrastructure
  is surfaced at the job level instead of hidden among unittest.skip
  lines. The docker job replaces its soft "Show environment" step; the
  firecracker job keeps its richer backend-status check.

Docs (tests/README.md, docs/ci.md) updated; unit coverage for the probes,
guards, and preflight in test_backend_skip_guards.py.

Closes #414

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/integration/test_sandbox_escape.py

@@ -31,7 +31,7 @@ from pathlib import Path
 from bot_bottle.backend import BottleSpec, get_bottle_backend
 from bot_bottle.bottle_state import cleanup_state
 from bot_bottle.manifest import ManifestIndex
-from tests._docker import skip_unless_docker_or_firecracker
+from tests._backend import skip_unless_selected_backend_available
 
 
 # Secrets planted in the bottle env as literals (agents substitute via
@@ -67,7 +67,7 @@ _DUMMY_HOST_KEY = (
 )
 
 
-@skip_unless_docker_or_firecracker()
+@skip_unless_selected_backend_available()
 @unittest.skipIf(
     os.environ.get("GITEA_ACTIONS") == "true"
     and os.environ.get("BOT_BOTTLE_BACKEND") != "firecracker",