docs(status-page): correct the command names in the README and e2e tests

[debidong]

What

The status page group is registered as status-page, but the README and the e2e suite still used statuspage, with subcommands (changes, create-incident, create-timeline, space-separated migrate ...) and a --id flag that never existed. Anyone following them — a person, or an agent reading the docs — got unknown command.

Everything here is corrected against the registrations in internal/cli/zz_generated_status_pages.go. No generated file is touched.

The two things that trip up a first call

Both are now documented explicitly:

• change-create takes <page-id> as a required positional that overrides the page_id key inside --data.
• the required updates array (with its nested component_changes) has no flag, so every real change-create carries a --data payload. --data - reads the whole body from stdin.

e2e

The suite is behind the e2e build tag and skips without FLASHDUTY_E2E_APP_KEY, so it was not failing CI — but it was broken in both directions. The positive tests could not have passed, and the negative ones passed for the wrong reason: asserting a validation error while actually getting unknown command.

Two further defects surfaced while fixing them:

• the list helpers unmarshalled a top-level array, but printGenericResult emits an {"items":[...]} envelope;
• the header assertion named columns the reflective heuristic never emits (status-page list has no displayColumns, so it takes the first 8 scalar fields).

Two tests had no valid replacement verb and were retargeted rather than deleted: list has no --id filter, so single-page lookup became status-page info, and the invalid-id case moved to change-active-list.

Every rewritten negative assertion was verified against the real binary and fails locally with no network.

Checks

go build ./..., go test ./... (9/9), go vet -tags e2e ./e2e/..., gofmt -l, golangci-lint run (0 issues), go run ./internal/cmd/skilldoc check (cards OK) — all clean.

Context

Part of the AI-SRE status page drafting work: the agent drafts a status page update and a human publishes it. The skill card the agent reads was already correct; these docs were the stale copy alongside it.