docs(test): record proton-pass-autofill as a deliberate permanent skip

The spec header framed un-skipping as a future "build a seeded-vault
fixture" TODO. That undersells it: the only way to satisfy the
`data-protonpass-*` assertion is a real UNLOCKED Proton vault, i.e. live
credentials, which are not committable to a CI-safe fixture. So the skip
is deliberate and indefinite — a manual / dev-profile-only check, not a
pending automation task.

Also record why keeping it skipped loses no automated coverage: the
compat-layer fixes behind causes 1-3 each have credential-free coverage
(sendMessage polyfill in runtime-external-polyfill.spec.ts; the bridge
window's mandatory sandboxing enforced by main/sandbox-enforcement.test.ts).
Only the aggregate live-vault end-to-end DOM assertion is unique here, and
it was verified by hand on the dev profile when cause 3 was fixed (d9acbf6d).

Comment-only; the spec stays test.describe.skip. peek 976a7700.