feat: distributed tracing — nest Claude Code under an inbound TRACEPARENT

[XinweiHe]

Draft for review. Adds W3C trace-context propagation so a backend that launches Claude Code (e.g. backend → sandbox → Claude Code) gets one connected trace, with the Claude Code session nested under the backend span.

How

• Reads TRACEPARENT (and BAGGAGE) from the hook process's environment and parents the ask-root spans under it (same trace_id).
• Standard OTEL propagation — TRACEPARENT env carrier is OTEP-0258; the backend builds it from get_current_trace_id()/get_current_span_id().
• Fail-open: no/malformed TRACEPARENT → fresh root trace, exactly as today.
• Baggage entries → traceroot.baggage.* attributes for correlation.
• Per-process scope: each claude invocation reads its own env, so reused/parallel-flow sandboxes stay separate (never a global).

Verified

• 87 unit tests (+4: nest-under-traceparent, fresh-trace-without, malformed-fail-open, baggage→attrs).
• End-to-end against prod: backend agent_task span → Claude Code Turn → LLM → tools, all one trace.

Notes for merge

• Behavioral change: now honors any TRACEPARENT in env (standard, but worth a glance).
• Recommend a version bump (feature) so installs detect the update, e.g. 0.1.0 → 0.2.0.
• Example backend (backend → sandbox → Claude Code) lives in the platform repo's examples/claude-code-distributed-tracing/.

🤖 Generated with Claude Code