feat(lint): doc-policy-lint reusable workflow, warning-only (lane 4)

[ArchonVII]

Context

Lane 4 of the document-policy rollout — epic ArchonVII/archon-setup#223, spec ArchonVII/archon-setup#224 (§5.4). Warning-first per the PR-template drift-guard precedent (#54).

Scope

• New reusable doc-policy-lint workflow, warning-only: status-header presence on durable docs/** (ADRs/fragments/own-format plans exempt), charter size budgets (OD4), dangling Supersedes/Superseded by links, placeholder tokens (TODO/TBD/N/A) in active docs.
• Promotion into consumers' .agent/check-map.yml docs category is explicitly deferred until a clean quarter of warnings — not in this lane.

Acceptance Criteria

• Warning-only verified: lint failures never fail the required gate.
• Scoped actionlint on changed workflow files.
• Permissions block audited; tag-ref alignment checked; integration test present (reusable-workflow review rule).

Deps: ArchonVII/repo-template#72 (charters define what is linted).

[ArchonVII]

Scope addition — §8.2 of the document-policy amendment (ArchonVII/archon-setup PR #262; epic #223).

The warning-only doc-policy-lint surface should carry the same two signals as the lane-3a checker (ArchonVII/repo-template #74), emitted as warnings, never blocking (consistent with the warn-first PR-template drift-guard precedent):

1. Index coherence — durable doc present but absent from its landing/index.
2. Stale active-doc terms — current-truth register changed in a PR while nearby active docs still carry stale tokens ("not deployed / next / remaining / deferred / blocked / pending"; issue/migration numbers).

No new lane — this rides the existing warn-only lint.