Add optimistic writeback flow with kanban drag-drop transitions

src/lib/writeback.ts

@@ -0,0 +1,56 @@
+import type { BeadIssue, BeadStatus } from './types';
+
+export type MutationStep =
+  | { operation: 'close'; payload: { id: string; reason?: string } }
+  | { operation: 'reopen'; payload: { id: string; reason?: string } }
+  | { operation: 'update'; payload: { id: string; status: 'open' | 'in_progress' | 'blocked' | 'deferred' } };
+
+function isBoardStatus(status: BeadStatus): status is 'open' | 'in_progress' | 'blocked' | 'deferred' | 'closed' {
+  return ['open', 'in_progress', 'blocked', 'deferred', 'closed'].includes(status);
+}
+
+export function planStatusTransition(
+  issue: Pick<BeadIssue, 'id' | 'status'>,
+  targetStatus: 'open' | 'in_progress' | 'blocked' | 'deferred' | 'closed',
+): MutationStep[] {
+  if (!isBoardStatus(issue.status) || issue.status === targetStatus) {
+    return [];
+  }
+
+  if (targetStatus === 'closed') {
+    return [{ operation: 'close', payload: { id: issue.id, reason: 'Moved to closed via board drag-and-drop' } }];
+  }
+
+  if (issue.status === 'closed') {
+    if (targetStatus === 'open') {
+      return [{ operation: 'reopen', payload: { id: issue.id, reason: 'Moved from closed via board drag-and-drop' } }];
+    }
+
+    return [
+      { operation: 'reopen', payload: { id: issue.id, reason: 'Moved from closed via board drag-and-drop' } },
+      { operation: 'update', payload: { id: issue.id, status: targetStatus } },
+    ];
+  }
+
+  return [{ operation: 'update', payload: { id: issue.id, status: targetStatus } }];
+}
+
+export function applyOptimisticStatus(
+  issues: BeadIssue[],
+  issueId: string,
+  targetStatus: 'open' | 'in_progress' | 'blocked' | 'deferred' | 'closed',
+  atIso: string = new Date().toISOString(),
+): BeadIssue[] {
+  return issues.map((issue) => {
+    if (issue.id !== issueId) {
+      return issue;
+    }
+
+    return {
+      ...issue,
+      status: targetStatus,
+      updated_at: atIso,
+      closed_at: targetStatus === 'closed' ? atIso : null,
+    };
+  });
+}