plane(1.7): gvisor tarball reachability documented as planned (manual, k3s)

docs/deployment.md B.1: Substrate's ateom-gvisor worker runs runsc from the
release tarball named in manifests/ate-install/sandboxconfig-gvisor.yaml
(gs://gvisor/releases/nightly/2026-09-02/<arch>/gvisor.tar.zstd, sha256-pinned
per architecture), fetched by the atelet on the node; the node must reach it or
a copied SandboxConfig must name a mirror with the same sha256. The check is the
atelet's fetch line in the worker pod's logs, run by hand on the day k3s exists;
labelled planned until then (design open question 3 stays open).

Gates: docs only; mkdocs build --strict passed with this paragraph present in
the 1.4 gate; no file a test or linter reads changed since the 1.5 gate.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

.kiro/specs/plane-on-kubernetes/tasks.md

@@ -55,7 +55,7 @@ go-ahead** with a ceiling; steps that need a hand on a cluster that does not exi
     in `tests/test_plane_reconciler.py:803` moves to 550 in task 2.4 with the reason in the assertion message.
   - Evidence: the numbers in `record.json`.
   - _Requirements: 2.1_
-- [ ] 1.7 gvisor tarball reachability (open question 3) **manual**
+- [x] 1.7 gvisor tarball reachability (open question 3) **manual** -- documented as planned in `docs/deployment.md` B.1; the live check waits for k3s
   - `docs/deployment.md` (B.1, planned): the k3s node must fetch
     `gs://gvisor/releases/nightly/2026-09-02/<arch>/gvisor.tar.zstd` (`substrate/manifests/ate-install/sandboxconfig-gvisor.yaml`)
     or a mirrored copy named in a copied SandboxConfig; the check is `kubectl -n ate-system logs` of the worker