feat: docs completeness rollout [1/4] — instructions #2361

Issue Description

Roll out content-completeness enforcement for instructions — the lowest-effort asset type — as the first increment of promoting authored-docs completeness from warning to hard-fail.

Scope:

• Introduce per-kind -RequireAuthoredContent scoping in scripts/linting/Validate-AssetDocs.ps1 (e.g., -RequireAuthoredContent instruction) so completeness can be promoted one asset type at a time. This capability is created here and reused by the prompts/skills/agents rollout issues.
• Backfill the authored When to use it section for every instruction docs page under docs/reference/instructions/** (~75 files). Instructions are non-interactive (applied via applyTo), so How to use it is N/A and Example usage is optional — this is the smallest authored surface.
• Flip the CI gate to hard-fail authored-completeness for kind: instruction only.

Additional Context

• Why first (lowest effort): ~75 files, but each needs only a short When to use it (often derivable from applyTo/description); no How-to and no Example prompt authoring.
• Depends on: feat: [3/7] asset-docs coverage/drift validator with Pester tests #2333 (validator), feat: [5/7] enforce asset docs in npm scripts and PR CI gate #2337 (CI gate), feat: [4/7] scaffold asset docs pages and wire Docusaurus Asset Catalog #2335 (pages scaffolded), refactor: unify asset-doc section contract into a single ordered section list #2468 (unified per-kind section list — land first so instruction's required/optional/N-A section semantics extend a single shared list instead of editing template + generator + validator in lockstep).
• Acceptance: validator fails when any instruction page's When to use it is still a stub; enforcement scoped to instructions only; per-kind flag documented.
• Follow-on to the "documentation as a required artifact" initiative (completeness rollout 1 of 4).