Keep agent streams running in the background

[KingParmenides]

Summary

• start the remote agent /stream/:taskId request from /api/agent using Next.js after() so the run continues after the initiating browser leaves the task page
• drain the background stream server-side while relying on the existing /api/agent/webhook + Pusher path for persisted messages and UI updates
• update the task page to stop owning the direct stream when the backend reports backgroundStream: true
• add an agent API regression covering the after()-scheduled stream request

Bounty: https://people.sphinx.chat/gobounties/id/4316

Verification

• npm_config_cache=/private/tmp/npm-cache-hive npm ci
• npm_config_cache=/private/tmp/npm-cache-hive npx eslint src/app/api/agent/route.ts 'src/app/w/[slug]/task/[...taskParams]/page.tsx' src/__tests__/integration/api/agent.test.ts (passes with existing warnings in task page)
• npm_config_cache=/private/tmp/npm-cache-hive npx vitest run src/__tests__/unit/api/agent-rollback-logging.test.ts
• npm_config_cache=/private/tmp/npm-cache-hive npm run build (passes with existing dependency/Redis warnings)
• git diff --check -- src/app/api/agent/route.ts 'src/app/w/[slug]/task/[...taskParams]/page.tsx' src/__tests__/integration/api/agent.test.ts

Note: the targeted integration test file cannot be executed in this local environment because TEST_DATABASE_URL/DATABASE_URL is not configured for integration tests.