Keep agent streams alive after navigation

[BenItBuhner]

Summary

• move agent stream draining into the /api/agent route with after() so the remote stream continues server-side after the browser receives session metadata
• stop returning the remote stream URL/token to the browser and let persisted webhook/Pusher messages drive UI updates
• clear the agent loading state when the persistent stream is finished and cover the Pusher finished event

Validation

• npm ci
• npx vitest run src/__tests__/integration/api/agent.test.ts src/__tests__/unit/lib/pusher.test.ts (unit suite selected by repo config: 28 passed)
• TEST_DATABASE_URL=postgresql://test:test@localhost:5433/hive_test DATABASE_URL=postgresql://test:test@localhost:5433/hive_test TEST_SUITE=integration npx vitest run src/__tests__/integration/api/agent.test.ts (3 passed)
• npm run lint -- --quiet
• git diff --check origin/master...HEAD

Note: npm run test:db:setup applied migrations but hit an existing setup-script cleanup error reading verificationToken.deleteMany from an undefined Prisma client model. The focused integration suite passed after setting both DB env vars directly.