Retire the smoke-10 chaos scenario; convergence is covered in-process

smoke-10 was a no-stressor 10-node tree-convergence sanity check (netem
off, no ping). Its subject, spanning-tree convergence and root election,
is now covered in-process, faster and deterministically, by the loopback
spanning-tree harness (src/node/tests/spanning_tree.rs: ring, star,
chain, 100-node and disconnected-component convergence) plus end-to-end
datagram delivery (src/node/tests/forwarding.rs). Real-UDP convergence
smoke still runs via static-mesh and the other scenarios' baseline
assertions, so no Docker coverage is lost.

Drop it from both runners in lockstep (the CHAOS_SUITES list and the
GitHub chaos matrix) so the parity guard stays green, delete the scenario
YAML, and update the chaos README.

.github/workflows/ci.yml

@@ -396,9 +396,6 @@ jobs:
             type: gateway
             topology: gateway
           # ── Chaos / stochastic scenarios ───────────────────────────────────
-          - suite: chaos-smoke-10
-            type: chaos
-            scenario: smoke-10
           - suite: churn-mixed-10
             type: chaos
             scenario: churn-mixed