Add Arize Phoenix LLM observability exporter

[whatever]

Summary

• Adds ArizeExporter(SpanExporter) that batches and ships spans to Arize Phoenix via OTLP/HTTP
• Auto-enabled when ARIZE_API_KEY + ARIZE_SPACE_ID env vars are set
• ObservabilityIntegration emits llm_call spans on every COST_UPDATE event with model, token counts, cost, and provider metadata
• Background worker thread handles batching/flush (non-blocking to agent pipeline)
• Graceful degradation: no-op when credentials absent, logged warnings on export failure

Stack

Part 4 of:

1. Wire context summarizer into hunter loop #127 — context shortener node
2. feat(sandbox): Add kubernetes sandbox support #126 — kubernetes sandbox
3. Add cwpro model_aliases routing and machine-mode parameters #130 — cwpro machine-mode
4. this PR — Arize observability

Configuration

Env Var	Purpose
ARIZE_API_KEY	Arize API key (required)
ARIZE_SPACE_ID	Arize workspace ID (required)
ARIZE_ENDPOINT	Override ingest URL (default: https://app.phoenix.arize.com)
ARIZE_PROJECT_NAME	Project name in Arize (default: "clearwing")
ARIZE_BATCH_SIZE	Spans per batch (default: 20)

Test plan

• Unit tests: OTLP format, env wiring, span emission, disabled state (7 tests)
• Integration: set env vars and run a sourcehunt to verify spans appear in Arize dashboard