Premise-coherence required-verdict forcing-function — validator + template + JSDoc (AC1 of #13144)

AC1 slice of #13144 (premise-coherence verdict in agent PR review). #13144 spans two independent slices — AC1 (this: the validator forcing-function) and AC2 (the SEEN-firewall AGENTS.md L1 trigger scope-widen, the separate higher-blast slice) — but agent-pr-body-lint mandates exactly one isolated Resolves #N per PR (the 1-PR-per-ticket model). So #13144 is the parent and this is the AC1 sub; PR #13738 Resolves this.

Scope (AC1)

Add a 4th required premise field — Premise Coherence — to the agent PR-review-body contract: the value-coherence verdict (should this premise exist at all, measured against the core values), forcing ARTICULATION (a specific verdict OR a scoped N/A — no value-surface for trivial PRs). Flip the premise snapshot from optional-during-migration to always-required (all four fields).

Acceptance Criteria

• PullRequestService validator requires the Premise Coherence anchor + flips the premise snapshot optional→always-required (4 fields).
• pr-review-template.md + pr-review-followup-template.md carry the **Premise Coherence:** field after Patch Verdict.
• pr-review-guide.md §0 + the manage_pr_review openapi description reflect the required-4 premise.
• A review omitting the field returns PR_REVIEW_TEMPLATE_VALIDATION_FAILED; unit-covered.

Out of Scope

• AC2: the SEEN-firewall AGENTS.md L1 trigger scope-widen (separate higher-blast slice, stays on parent #13144).

Related

• Parent: #13144 (force a premise-coherence verdict in agent PR review)
• PR: #13738 (the implementation, 47 specs green)