Hygiene: comment budget, shared request helpers, dead client verbs, two fixes

[CMGS]

Four commits over the extensions controllers, pkg/e2bcompat, pkg/sandboxd, the examples and the tagged harnesses. No file under pkg/scale, controllers/ or api/ is touched.

review: comment budget — 83 comment lines out, 2 in. Numbered section labels, restated code, the #1107 adoption rationale written four times, the triggeredAdoptionEntry and ensureClaimIdentityLabels docs repeated at their call sites, the duplicated SandboxTemplate -> claims TODO, and 17 numbered step-narration comments in the lifecycle example. The spaced // nolint:revive directives were inert prose (golangci-lint only honours //nolint); lint stays at 0 issues. on both GOOS without them.

review: named signatures and shared helpers

• staleCheck bundles the four template-state parameters isSandboxStale took over 8 lines; buildSandboxCR drops poolNameHash, which it derives from the pool name the same way the caller does.
• One decodeBody / decodeOptionalBody pair replaces six copies of the decode-or-400 block. The required/optional split is deliberate: createSandbox and setTimeout must keep rejecting an absent body, which decodeOptional accepted.
• Client.sendNoBody replaces the request/auth/drain/status skeleton in Release, DeleteCheckpoint and sandboxVerb, keeping each call's error text and accepted statuses.
• pollVisible (examples) and pollDetail (e2e) replace seven hand-rolled deadline loops; the six must() forwarders now call benchutil.Must directly.

cut: uncalled client verbs — ClaimCheckpoint and Promote (with CheckpointClaimSpec, PromoteSpec, PromoteResult) have no caller in sandbox-operator, vk-sandbox, instar or sandbox; the grep is in the commit body. Client.Sandboxes and SandboxSummary were on the same list but are live: vk-sandbox reaches them from provider.go, create.go, orphan.go and inventory/publisher.go, so they stay.

fix: — version.Print panicked on a template error rather than returning it, and the collectors were registered from a var-initializer IIFE, an init() in disguise that ran in every binary importing the package. Register() is now called once from the operator's main, the only process serving a controller-runtime metrics endpoint. Both have regression tests.

Gates

All with GOWORK=off:

$ make lint
==> golangci-lint GOOS=linux
0 issues.
==> golangci-lint GOOS=darwin
0 issues.

$ make fmt-check     # exit 0
$ asl ./...          # exit 0, no findings
$ GOOS=linux asl ./...  # exit 0, no findings
$ make vet-tagged    # all 7 tagged harnesses type-check
$ go test -race -count=1 ./...   # every package ok