runbook: record how the image gate was taken to zero findings

trivy on the worker image: 39 findings (2 CRITICAL) -> 18 -> 5 -> 0. Verified in CI
run #7 on 4a426db: 'Total: 0 (HIGH: 0, CRITICAL: 0)' for all three images.

The last five lived in kubectl's vendored golang.org/x/net and Go stdlib, inside the
newest kubectl published. No version cleared them; removing the binary did.

RUNBOOK.md

@@ -78,7 +78,26 @@ type=gha` are both no-ops.
 | The pipeline retriggers itself forever | The `bump` job commits to the repo it is triggered by. | `[skip ci]` in the commit message (Gitea honours it), **and** a `git diff --quiet` guard so an unchanged digest commits nothing. |
 | Service container unreachable at `localhost` | Jobs run *inside* a container, so a service is reached by its **service name**, not localhost. | `postgres:5432`, not `localhost:5432`. |
 
-### 4. Verify the whole loop, not just the green checkmarks
+### 4. Keeping trivy green
+
+The image gate is a moving target: trivy's vulnerability DB updates daily, so an image that
+passed yesterday fails today without a single line of code changing. Two rules keep it sane.
+
+**Bump the version, do not add an ignore.** The worker image went 39 findings (2 CRITICAL)
+→ 18 → 5 → 0 across three fixes, each a version bump or a removal:
+
+| Change | Result |
+|---|---|
+| `alpine/helm` 3.16.2 → 3.21.3, `kubectl` 1.31.2 → 1.35.3 | 39 → 18, both CRITICALs cleared |
+| `kubectl` 1.35.3 → 1.36.2 (k8s 1.35.x vendors spdystream 0.5.0; the fix is 0.5.1) | 18 → 5 |
+| dropped kubectl entirely — `helm --create-namespace` replaced `kubectl apply` | **5 → 0** |
+
+**The cheapest CVE is the binary you do not ship.** The last five findings lived in
+kubectl's vendored `golang.org/x/net` and Go stdlib, inside the newest kubectl that exists —
+no version cleared them. kubectl was in that image for exactly one call, and helm already
+does the same thing with a flag. Removing it removed the CVEs, a binary, and an adapter.
+
+### 5. Verify the whole loop, not just the green checkmarks
 
 ```bash
 # the digest CI pushed