Decide whether AttemptDB receives OpenTelemetry (ADR 0003)

[nullarch]

Hooks give AttemptDB the execution lifecycle: sessions, prompts, tool calls, permissions, stops, subagents. They do not carry what the agents' OpenTelemetry exporters carry: model name per response, input/output/cache token counts, cost, API latency and errors.

So "complete agent timeline" today means hooks + git context, and every cost or model question is unanswerable.

docs/adr/0003-otel-intake.md proposes receiving OTLP/HTTP JSON on the daemon at 127.0.0.1:4318, loopback only, off by default, mapped into canonical events under the capture mode, with the installer registering OTEL_EXPORTER_OTLP_ENDPOINT using the same structural-edit safety as hooks. gRPC and protobuf are deliberately not proposed: they would cost a dependency the single-binary rule does not want.

The decision is open, and that is what this issue tracks. Useful input:

• Do your agents emit OTLP today, and in which encoding?
• Would you want token/cost per turn on the timeline enough to run a local receiver?
• Is there a provider whose telemetry shape we should fixture before committing?

Read the ADR and comment. Implementation issues will be split out once it is accepted or rejected.