feat(telemetry): add OpenTelemetry tracing for workflow execution

[hertznsk]

Closes#481

This PR introduces opt-in OpenTelemetry tracing for Conductor.

What is added

• New optional runtime.telemetry YAML block with enabled and service_name fields.
• New conductor.telemetry package with lazy SDK imports, OTLP tracer provider setup, and a TelemetrySubscriber that turns workflow events into detached OpenTelemetry spans.
• Orchestration spans for workflows, agents, parallel groups, for-each items, subworkflows, tools, validators, and human gates.
• Native Pydantic AI instrumentation for claude and openai providers, correlated with Conductor runs via gen_ai.conversation.id.
• CLI wiring in conductor run and conductor resume, including the resume-with-dashboard path.
• conductor validate warning when telemetry is enabled but the SDK extra is missing.
• Documentation, example workflow, changelog entry, and architecture notes.

Verification

• make check passes (ruff + ty).
• make test passes: 7653 passed, 58 skipped, 14 deselected.
• make test-frontend passes: 151 passed.
• make validate-examples passes, including the new examples/telemetry.yaml.

Commits

• build(deps): add opentelemetry-api core dep and telemetry extra
• feat(telemetry): add telemetry package skeleton with config model and SDK guards
• feat(telemetry): add event-to-span TelemetrySubscriber with detached-span lifecycle
• feat(telemetry): wire TelemetrySubscriber into run and resume CLI paths
• feat(config): add runtime.telemetry block with validate-time SDK warning
• feat(providers): enable native pydantic-ai instrumentation with run correlation when telemetry is active
• feat(engine): include item index in for_each per-item event payloads
• test(telemetry): add regression suite for failure paths, resume, and export degradation
• docs(telemetry): document OpenTelemetry tracing, add example workflow
• docs(agents): document telemetry package in architecture overview

[hertznsk]

I mark it with a draft to test it.