feat: add fusion for multi-run sessions

Adds Run fusion for multi-run-like sessions
Combines sibling outputs into a new fusion session
Adds configurable Fusion prompts in Magic Prompts settings

packages/ui/src/lib/multirun/title.ts

@@ -0,0 +1,42 @@
+export type ParsedMultiRunTitle = {
+  groupSlug: string;
+  providerID: string;
+  modelID: string;
+  index?: number;
+  fusion: boolean;
+};
+
+const GROUP_SLUG_PATTERN = /^[a-z0-9](?:[a-z0-9-]{0,48}[a-z0-9])?$/;
+
+export const parseMultiRunSessionTitle = (title?: string | null): ParsedMultiRunTitle | null => {
+  if (!title) return null;
+  const segments = title.split('/');
+  if (segments.length !== 3 && segments.length !== 4) return null;
+
+  const [groupSlug, providerID, modelID, suffix] = segments;
+  if (!GROUP_SLUG_PATTERN.test(groupSlug)) return null;
+  if (!providerID?.trim() || !modelID?.trim()) return null;
+  if (providerID !== providerID.trim() || modelID !== modelID.trim()) return null;
+
+  if (segments.length === 3) {
+    return { groupSlug, providerID, modelID, fusion: false };
+  }
+
+  if (suffix === 'fusion') {
+    return { groupSlug, providerID, modelID, fusion: true };
+  }
+
+  if (!/^\d+$/.test(suffix)) return null;
+  const index = Number.parseInt(suffix, 10);
+  if (!Number.isSafeInteger(index) || index <= 0) return null;
+
+  return { groupSlug, providerID, modelID, index, fusion: false };
+};
+
+export const isMultiRunSessionTitle = (title?: string | null): boolean => {
+  return parseMultiRunSessionTitle(title) !== null;
+};
+
+export const getFusionSessionTitle = (groupSlug: string, providerID: string, modelID: string): string => {
+  return `${groupSlug}/${providerID}/${modelID}/fusion`;
+};