fix(gateway): persist git-gate state across gateway restarts

Per-bottle git-gate state (bare repos under /git/<id>, deploy creds under
/git-gate/creds/<id>) was provisioned once at bottle launch and lived only
in the gateway's ephemeral storage. A gateway rebuild/restart wiped it and
nothing re-provisioned already-running bottles, so their agents 404'd on
fetch/push. Same class of bug as the CA (#510); the orchestrator restores
only egress tokens, not git-gate declarations.

Persist the state on both backends, mirroring the CA-persistence approach:

- firecracker: attach a second persistent data drive (/dev/vdc) to the
  gateway VM and bind-mount its git/ + creds/ subdirs onto /git and
  /git-gate/creds in the gateway guest init, before the data plane starts.
  Generalize the VM config to a stable-ordered data_drives tuple (CA=vdb,
  git=vdc; orchestrator registry stays vdb).
- docker: bind-mount host dirs (host_gateway_git_dir / creds_dir, under the
  never-pruned app-data root) onto /git and /git-gate/creds, with
  BOT_BOTTLE_DOCKER_GIT_MOUNT / _CREDS_MOUNT env overrides so CI isolates
  them to per-run volumes it cleans up.

Teardown already rm -rf's /git/<id> + creds, so the persistent store
self-cleans over the normal lifecycle.

Closes #512

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

.gitea/workflows/pre-release-test.yml

@@ -100,6 +100,8 @@ jobs:
           export BOT_BOTTLE_DOCKER_CLIENT_NETWORK="$DOCKER_CLIENT_NETWORK"
           export BOT_BOTTLE_DOCKER_ROOT_MOUNT="bot-bottle-ci-root-$RUN_KEY"
           export BOT_BOTTLE_DOCKER_CA_MOUNT="bot-bottle-ci-ca-$RUN_KEY"
+          export BOT_BOTTLE_DOCKER_GIT_MOUNT="bot-bottle-ci-git-$RUN_KEY"
+          export BOT_BOTTLE_DOCKER_CREDS_MOUNT="bot-bottle-ci-creds-$RUN_KEY"
           python3 -m coverage run -m scripts.unittest_gate \
             -t . -s tests/integration -v \
             --minimum-executed 22 --fail-on-skip
@@ -110,7 +112,9 @@ jobs:
           RUN_KEY="${GITHUB_RUN_ID:-${GITHUB_RUN_NUMBER:-0}}"
           docker volume rm --force \
             "bot-bottle-ci-root-$RUN_KEY" \
-            "bot-bottle-ci-ca-$RUN_KEY" 2>/dev/null || true
+            "bot-bottle-ci-ca-$RUN_KEY" \
+            "bot-bottle-ci-git-$RUN_KEY" \
+            "bot-bottle-ci-creds-$RUN_KEY" 2>/dev/null || true
 
       # Non-dot name so upload-artifact's dotfile-skipping glob picks it up.
       - name: Stage docker coverage for upload