ci: reuse backend.has_backend / backend status for readiness

Review feedback: the hand-rolled `/dev/kvm` + docker-daemon capability
probes duplicated logic the CLI already owns. Delegate instead.

- tests/_backend.py skip guards now gate on `bot_bottle.backend.has_backend`
  (each backend's `is_available()` classmethod — the same probe behind
  `./cli.py backend status`), dropping the bespoke `Capability` probes.
- Remove tests/backend_preflight.py; the docker integration job runs
  `./cli.py backend status --backend=docker` as its preflight (clear
  per-check summary, non-zero exit when unready), matching the firecracker
  job. The firecracker preflight reverts to its original binary/KVM checks
  (backend status doesn't cover those).
- Unit test + docs updated to match.

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

.gitea/workflows/test.yml

@@ -105,13 +105,14 @@ jobs:
 
       # Fail loudly if the backend this job promises isn't actually usable,
       # rather than letting every test silently `unittest.skip` and the job
-      # go green on zero coverage. Same capability probe the skip guards use.
+      # go green on zero coverage. `backend status` prints a clear per-check
+      # summary (docker on PATH, daemon reachable) and exits non-zero when a
+      # prerequisite is missing — the same readiness check the skip guards
+      # gate on via `has_backend`.
       - name: Preflight — Docker backend is ready
-        env:
-          BOT_BOTTLE_BACKEND: docker
         run: |
           python3 --version
-          python3 -m tests.backend_preflight docker
+          python3 cli.py backend status --backend=docker
 
       - name: Run integration tests (docker) with coverage
         env:
@@ -158,13 +159,10 @@ jobs:
         uses: actions/checkout@v4
 
       - name: Preflight — Firecracker host is ready
-        env:
-          BOT_BOTTLE_BACKEND: firecracker
         run: |
-          # Standardized capability line — same probe (and PASS/FAIL wording)
-          # the integration skip guards use, so a missing backend is visible
-          # at the job level rather than hidden among `unittest.skip` lines.
-          python3 -m tests.backend_preflight firecracker
+          command -v firecracker >/dev/null || {
+            echo "firecracker not on PATH — provision the runner (README: Firecracker on Linux)"; exit 1; }
+          test -e /dev/kvm || { echo "/dev/kvm missing — KVM not available on this runner"; exit 1; }
           # `backend status` exits non-zero unless the TAP pool is up + no
           # range overlap; it prints the exact `backend setup` fix.
           python3 cli.py backend status --backend=firecracker