refactor(observability): remove eval OTLP export

[christso]

Summary

• remove AgentV eval-run OTLP export CLI flags/config and exporter plumbing
• delete the core observability exporter modules, OTel optional dependencies, and Langfuse export example
• update docs/ADRs/plans to state SUT/provider emits telemetry directly and AgentV correlates via external_trace metadata

Verification

• bun test packages/core/test/evaluation/config.test.ts packages/core/test/evaluation/loaders/config-loader.test.ts apps/cli/test/commands/trace/trace.test.ts packages/core/test/evaluation/trace-envelope.test.ts apps/cli/test/commands/eval/artifact-writer.test.ts
• bun --filter @agentv/core build
• bun --filter agentv typecheck
• CLI smoke: eval run help has no OTel flags; --otel-file is rejected as unknown

Bead: av-mey9

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