feat(phoenix): add otel backend preset

[christso]

Summary

Adds the first Phase A slice from the Phoenix integration completion plan:

• Adds a phoenix OTel backend preset for normal agentv eval runs.
• Documents Phoenix as two surfaces: live OTLP trace export and the repo-local dataset/experiment adapter.
• Updates CLI help, Phoenix adapter support notes, e2e notes, and the AI-facing config schema reference.

Handoff Tracking

Related parent tracker: #1281

Follow-up subissues:

• feat(phoenix): complete deterministic adapter parity #1282 - Deterministic adapter parity
• fix(phoenix): make full dry-run structural parity actionable #1283 - Full dry-run structural parity
• feat(phoenix): run real AgentV targets inside Phoenix experiments #1284 - Real AgentV target execution inside Phoenix experiments
• feat(phoenix): support AgentV-authoritative LLM graders and rubrics #1285 - AgentV-authoritative LLM graders and rubrics
• feat(phoenix): support trace and metric graders through Phoenix trace IDs #1286 - Trace and metric graders through Phoenix trace IDs
• chore(phoenix): decide and implement adapter package publishing posture #1287 - Adapter package publishing posture

This PR does not close #1281. It lands the plan plus the first supported Phoenix OTel export slice.

Details

The preset defaults to local Phoenix at http://localhost:6006/v1/traces, normalizes PHOENIX_COLLECTOR_ENDPOINT without duplicating /v1/traces, adds bearer auth from PHOENIX_API_KEY, and sets x-project-name from PHOENIX_PROJECT_NAME or PHOENIX_PROJECT when present.

Validation

• bun test test/observability/otel-exporter.test.ts in packages/core: 29 pass
• bun --filter @agentv/core typecheck
• bun --filter @agentv/core build
• bun --filter agentv typecheck
• bun --filter @agentv/web build
• bun run lint
• bun apps/cli/src/cli.ts eval run --help | rg -n "otel-backend|phoenix|export-otel"
• Pre-push hook passed on git push: build, typecheck, lint, test, and example YAML validation

Notes

Live Phoenix UAT was not run in this session because it requires a running Phoenix server. The docs and e2e notes describe the command to use for local or hosted Phoenix verification.

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