Keep the scratch flag out of the Core-generated contracts

Why: src/contracts.ts is regenerated from Agent-System-Core by the Dev
composite; the scratch field added there is dropped on regeneration and
tsc then fails in make build.

Evidence: contracts.ts equals the Core-generated output; npx tsc -b
--noEmit, npx vitest run (685 tests) and check-knip pass locally (SELF).

Impact: ConversationView (Conversation & { scratch?: boolean }) in
api.ts carries the server-local flag, mirroring the Backend
conversationView; no behavior change.

Refs: Agent-System-Frontend#123, Agent-System-Dev#691;
agent-exec receipt u3f-run (cursor, success)

Co-authored-by: Cursor <cursoragent@cursor.com>

test/scratch-session.test.ts

@@ -1,16 +1,17 @@
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import { auth } from '../src/auth'
 import { canCreateProject, store } from '../src/store'
-import type { Conversation, Project } from '../src/contracts'
+import type { ConversationView } from '../src/api'
+import type { Project } from '../src/contracts'
 
 const project = (id: string): Project => ({ id, name: id, createdAt: '', updatedAt: '' })
-const task = (id: string, projectId: string, scratch = false, updatedAt = ''): Conversation =>
+const task = (id: string, projectId: string, scratch = false, updatedAt = ''): ConversationView =>
   ({ id, projectId, title: id, createdAt: '', updatedAt, ...(scratch ? { scratch: true } : {}) })
 const response = (value: unknown, status = 200) => new Response(JSON.stringify(value), { status })
-const detail = (conversation: Conversation) => response({ conversation, messages: [], runs: [], activeHitl: [], lastSeq: 0 })
+const detail = (conversation: ConversationView) => response({ conversation, messages: [], runs: [], activeHitl: [], lastSeq: 0 })
 
 let projects: Project[]
-let tasks: Conversation[]
+let tasks: ConversationView[]
 let me: Record<string, unknown>
 let requests: { path: string; init?: RequestInit }[]
 let scratchCount: number