7c6c906db2
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>