feat: support nested traces under external Weave parent ops

[aajais]

Summary

• Forwards WEAVE_PARENT_CALL_ID and WEAVE_TRACE_ID environment variables from the hook handler into the daemon event payload, enabling Claude Code sessions invoked inside a @weave.op to be recorded as child calls.
• Uses the parent's trace ID (when present) instead of creating a new standalone root trace, and sets parent_id on the session call to form a proper parent-child hierarchy in Weave.

Changes

hooks/hook-handler.sh

• Reads WEAVE_PARENT_CALL_ID and WEAVE_TRACE_ID from the environment and injects them into the JSON event payload before sending to the daemon socket.

src/daemon.ts

• Adds parentCallId to the SessionState interface.
• Extracts weave_parent_call_id and weave_trace_id from the incoming event payload.
• Prefers the parent trace ID over the auto-resolved trace ID when available.
• Sets parent_id on the claude_code.session call to the parent call ID (instead of null), nesting the session under the external op.

Test plan

• Run a Claude Code session without WEAVE_PARENT_CALL_ID / WEAVE_TRACE_ID set.

• Run a Claude Code session with both env vars set (e.g. from within a @weave.op)

[github-actions]

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[chance-wnb]

Hey Aayush! Thanks for your contribution! I love it!

I made one suggestion, could you please take a look?

[chance-wnb]

1. I think JSON handling in the form of string manipulation is fragile. can we avoid it if we can?

2. I think the daemon is capable of reading the same env vars, why don't you just have the daemon process read it directly instead of getting passed over?

[chance-wnb]

Also @aajais could you please sign the CLA?