feat: add TraceData.headerContent? for pre-emoji header access

`withTraceNodeBefore` prepends `TraceResult.toEmoji` to the header
before storing it in the trace tree. This makes it difficult for
programmatic consumers to compare headers across trace runs, since
the same check gets different emoji prefixes depending on
success/failure.

Add `headerContent? : Option MessageData` to `TraceData` that stores
the original pre-emoji header content. This is purely additive — the
emoji-prefixed header continues to be stored as before, and all
existing trace output is unchanged.

Closes #13069

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>