#26: optional proposed-use-case field on propose-RFC + propose-PR

Adds an optional "What will you be using this for?" capture as a sibling
to the required justification on both propose surfaces, per roadmap #26.

- propose-RFC modal (ProposeModal): optional textarea below the required
  "Why is this RFC needed?" pitch, labeled "What will you be using this
  RFC for? (optional)".
- propose-PR modal (PRModal): optional textarea below the required
  description, labeled "What will you be using this change for?".
- Backend: ProposeBody / OpenPRBody gain an optional `proposed_use_case`
  (NULL/omitted accepted, no min, 8000-char cap matching the existing
  free-text bound). Persisted to a new canonical side table
  `proposed_use_cases` keyed by PR number, mirrored onto the cache
  columns added by migration 021. Returned on the proposal list/detail,
  RFC detail (by slug), and PR detail endpoints.
- Display: ProposalView, RFCView (main only), and PRView render the
  captured use case with a muted "left blank" treatment when NULL.
- migration 021: nullable `proposed_use_case` on cached_rfcs/cached_prs
  plus the reconcile-proof `proposed_use_cases` truth table.
- New vertical test_proposed_use_case_vertical: persists+returns when
  supplied, accepted as NULL/omitted, for both surfaces.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/PRModal.jsx

@@ -15,6 +15,8 @@ import { EVENTS, track } from '../lib/analytics'
 export default function PRModal({ slug, branch, branchIsPrivate, onClose, onOpened }) {
   const [title, setTitle] = useState('')
   const [description, setDescription] = useState('')
+  // #26: optional ground-truth use case for this change.
+  const [useCase, setUseCase] = useState('')
   const [drafting, setDrafting] = useState(true)
   const [submitting, setSubmitting] = useState(false)
   const [confirmed, setConfirmed] = useState(!branchIsPrivate)
@@ -39,7 +41,11 @@ export default function PRModal({ slug, branch, branchIsPrivate, onClose, onOpen
     setSubmitting(true)
     setError(null)
     try {
-      const { pr_number } = await openPR(slug, branch, { title: title.trim(), description: description.trim() })
+      const { pr_number } = await openPR(slug, branch, {
+        title: title.trim(),
+        description: description.trim(),
+        proposedUseCase: useCase.trim() || null,
+      })
       // v0.15.0 — analytics: fire on §10.2 PR-open success. slug
       // and pr_number are the join keys; title/description stay out.
       track(EVENTS.PR_OPENED, { rfc_slug: slug, pr_number })
@@ -104,6 +110,20 @@ export default function PRModal({ slug, branch, branchIsPrivate, onClose, onOpen
             what was argued, what shifted, what the arbiters are asked
             to consider.
           </p>
+          <label className="modal-label">What will you be using this change for? (optional)</label>
+          <textarea
+            className="modal-textarea"
+            value={useCase}
+            onChange={e => setUseCase(e.target.value)}
+            placeholder="The concrete thing this change unlocks for you. Optional."
+            disabled={drafting || submitting}
+            rows={3}
+            maxLength={8000}
+          />
+          <p className="field-help">
+            #26: the concrete ground-truth use case — distinct from "why
+            it's needed" above. Leave blank if you'd rather not say.
+          </p>
           {error && <p className="field-error">{error}</p>}
         </div>
         <div className="modal-actions">