feat(agent): have sub-agents stream their responses to keep the tunnel alive

[OisinKyne]

Problem to be solved

Cloudflare tunnels timeout on 100s of no data. Streaming an agents response may resolve that, but it also may put more complexity on the client side caller, we should consider if we need to improve paybe buy.py pay command to handle it if we haven't already. (I think its dealt with at the verifier side)

Summary

obol-stack/:

• internal/x402/verifier.go — statusRecorder.Flush() + Hijack()
  so SSE chunks actually stream (the bug from the previous
  turn).
• internal/x402/verifier_test.go —
  TestVerifier_HandleProxy_StreamsSSEChunks (over-the-wire pacing
  assertion) + TestVerifier_HandleProxy_NonStreamingResponse.
• CLAUDE.md:
  • Added agent to obol sell and a new obol buy inference row
    in the CLI tree.
  • Added an "Agent-backed offers" paragraph and a "Prefer
    stream: true for OpenAI-compatible paid endpoints" block in the
    Monetize Subsystem, explicitly calling out the Cloudflare
    tunnel ~100s idle window and citing the test as the regression
    sentinel.
  • Added obol sell agent and obol buy inference to the CLI
    flag-reference line.

[bussyjd]

Superseded by the consolidated integration PR #598, which merges this branch together with the other PRs ≥ #593 into integration/pr-593-plus for a single reviewable diff. Review hardening fixes ride in #599 (targets the integration branch). Closing in favour of the consolidated review — reopen if you'd prefer to land this one standalone.