Add runtime event client boundary for web React (issue #3)

[enricopiovesan]

Summary

• Add a thin spec-033 HTTP client (traverseClient) for execute, poll, and trace fetch
• Add useExecution hook driving UI state through idle → loading → polling → succeeded/failed
• Wire traverse-starter App.tsx to start workflows and display runtime output + trace events
• Fix health check to /healthz and align env vars with docs/traverse-runtime.md

Closes #3.

Definition of Done

• React can start a workflow through a public Traverse runtime/client boundary
• React subscribes to correlated runtime events (trace fetch after execution)
• UI state is driven by events (loading, polling, succeeded, failed)
• Failure, loading, progress, and final states are visible in the UI
• No private Traverse internals imported

Validation

npm run typecheck
npm run lint
npm run test
bash scripts/ci/coverage_gate.sh

All pass locally (14 tests).

Test plan

• Unit tests for createTraverseClient (execute, poll, trace, error paths)
• Unit tests for useExecution (async poll, sync execute, failure paths)
• App integration test: submit note → poll → render runtime output + trace event
• Manual: start Traverse v0.3.0 runtime, run npm run dev, submit note, confirm live execution flow

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