Prototype agent lifecycle events (semconv #445) across LangGraph, CrewAI and ADK

On open-telemetry/semantic-conventions-genai#445, lmolkova asked for real instrumentation over two or three libraries in this repo as evidence for the candidate gen_ai.agent.paused / gen_ai.agent.checkpointed / gen_ai.agent.resumed events, instead of hand-emitted reference scenarios.

Plan, one PR each:

• LangGraph: extend the langchain instrumentation. Draft up as #536.
• CrewAI: fill in the existing skeleton package via CrewAI's event bus (flow persistence, human feedback, checkpoints).
• ADK: ADK ships its own OpenTelemetry tracing, so this would be a plugin rather than an instrumentation package. Open to guidance on whether it belongs in this repo at all.

Candidate event names stay pending the semconv proposal, the same approach as #507 and semantic-conventions-genai#483. Flagging before starting the CrewAI and ADK work in case the direction or the split should differ.

Start by reading semantic-conventions-genai#445, #507, semantic-conventions-genai#483, and the LangGraph draft #536. Then inspect the existing CrewAI skeleton and determine how its event bus can expose persistence, human feedback, and checkpoint events. Done requires an agreed split and direction, plus real CrewAI instrumentation; ADK scope remains an open plugin decision.