[codex] Add k6 SSE stream soak test

[MyouzzZ]

Summary

• Adds load/agent-stream.js, a k6 k6/x/sse soak test for many concurrent /agent/stream clients.
• Sends pause/resume broadcasts during the soak and gates delivery, clean disconnects, broadcast latency, and connection errors with thresholds.
• Exposes an agent_sse_clients Prometheus gauge so the test can verify SSE connections return to baseline.
• Serializes SSE payloads defensively so circular payloads cannot crash broadcastSSE.
• Documents pnpm load:stream and the tunable environment variables.

Closes #804

Validation

• node --check load/agent-stream.js
• node -e "JSON.parse(require('fs').readFileSync('package.json','utf8')); console.log('package json ok')"
• Targeted TypeScript check no longer reports agent/server.ts; it is still blocked by existing errors in shared/bill-audit.ts, shared/rate-limit.ts, and shared/redis.ts.
• Live k6 soak was not run locally because k6 is not installed in this environment.

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