ci(openspec): gate archived changes on having no unticked tasks

[Harol-Reina]

What

One step beside the existing strict validate, in the same openspec job:

- name: Validate archived changes (no unticked tasks)
  run: npx -y @fission-ai/openspec@1.13.1 validate --archived --no-interactive

Why now, and not before

ADR-0023 §7 deferred this gate per repo until that repo reached rc=0 — wiring it earlier would have turned existing debt into a red main. This repo reached rc=0 on 2026-09-21. Before that it carried closed changes with work still outstanding, and nothing could see it: validate --all --strict does not read changes/archive/.

It is a gate, not decoration

Proved on a scratch copy rather than assumed:

control (untouched)                          rc=0
one box unticked in one archived change      rc=1

What this costs from here

Archiving a change with an unticked box now fails CI. That is the intent — the alternative is what this repo just spent a session undoing — but it adds real friction at close-out, and the box must then be dispositioned honestly: - [x] with evidence, or harvested into an open change. Never ticked to get green.

Verification on this branch

openspec validate --archived      13 passed, 0 failed
openspec validate --all --strict   6 passed, 0 failed
npm run lint:generated-types      ratchet OK (floor 37)

decision_ref: verbara-meta/ADR-0023 §7

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