docs(review_prs): fix Stage-3 cross-ref and require rationale in result schema (#621)

Address two review nits on the /review_prs sketch:

- The agent-prompt parenthetical cited `review_pr` for the submission
  mechanism, but `review_pr.md` tops out at Stage 5 "Present to User" and
  documents no `gh api` path. The POST-to-/reviews mechanism lives in this
  file's own Stage 3 — re-anchor the cross-reference there.
- Add `rationale` to the RESULT schema's `required` list so an agent can no
  longer satisfy the schema with an empty rationale, which the prose
  (L135-136) and the Stage-3 report table both assume is present.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: bgagent

.abca/commands/review_prs.md

@@ -145,7 +145,7 @@ export const meta = {
   description: 'Review a set of PRs, one review_pr per PR, return compact verdicts',
   phases: [{ title: 'Review' }],
 }
-const RESULT = { type: 'object', required: ['number', 'verdict', 'reviewUrl'], properties: {
+const RESULT = { type: 'object', required: ['number', 'verdict', 'rationale', 'reviewUrl'], properties: {
   number: { type: 'number' },
   verdict: { enum: ['APPROVE', 'COMMENT', 'REQUEST_CHANGES'] },
   rationale: { type: 'string' },
@@ -157,7 +157,7 @@ const results = await parallel(args.map((pr) => () =>
     `suggestions. Worktree: ${pr.worktreePath}. Diff: ${pr.diffPath}. commit_id: ${pr.headSha}. ` +
     `Base: ${pr.baseRef}${pr.baseRef !== 'main' ? ' (STACKED — diff is vs the base branch)' : ''}. ` +
     `Governance: gate is an approved backing issue; a pr/* branch is a waived nit. Read existing ` +
-    `review threads first and verify prior blocking claims. Submit via gh api (see review_pr).`,
+    `review threads first and verify prior blocking claims. Submit via gh api (see Stage 3).`,
     { label: `review:#${pr.number}`, phase: 'Review', schema: RESULT },
   )))
 return results.filter(Boolean)