Add NullWatch Flight Recorder observability cockpit

[venzeles]

Adds a local-first Agent Flight Recorder demo path for the nullclaw ecosystem. NullWatch can now provide deterministic local traces/evals for demos, and NullHub can proxy and visualize those runs through a new Observability page.

This gives contributors a one-command local observability story without external services or secrets.

What changed:

• Registers nullwatch as a known NullHub component.
• Adds /api/observability/* reverse proxy support via NULLWATCH_URL and optional NULLWATCH_TOKEN.
• Adds a Flight Recorder page at /observability for run summaries, span timelines, evals, cost, tokens, and error context.
• Adds API client methods for NullWatch summary, runs, spans, evals, and health.
• Adds screenshots for PR/demo review.
• Updates README and hackathon submission notes.

Validation performed:

• zig build test -Dembed-ui=false --summary all
• npm --prefix ui run build
• zig build test --summary all
• Local smoke test through NullHub -> NullWatch proxy:
  • /api/observability/v1/summary
  • /api/observability/v1/runs?limit=20