docs(snapshot): remove stale prompt scrubber conflict residue

Delete the leftover diff3 parent marker and both copies of the obsolete request-header prompt JSDoc. Keep only the current system/message tokenization contract. This is a local comment-only finding; neither oracle behavior nor fixture content changes.

Evidence: git diff --check is clean; focused normalize.spec.ts scrubSystemPrompts test passes (1 selected, 64 skipped). The earlier header-preservation commit intentionally retained this marker so the findings stay independent.

packages/test-support/session-snapshot/src/normalize.ts

@@ -517,19 +517,6 @@ function hasSessionFormatVersion(rawLog: string): boolean {
 }
 
 /**
- * Replace system-prompt content in request headers with `{{system}}` tokens
- * while retaining field presence.
- * Other header content stays verbatim, so a header-pinning fixture can keep
- * its complete tool schemas while every JSONL fixture omits the prompt text.
- * Lines without a system payload pass through byte-for-byte; the transform is
- * idempotent.
-||||||| parent of 842fdd6a51 (refactor(session): log the system prompt as surface node 0)
- * Replace system-prompt content in request headers with `{{system}}` tokens
- * while retaining field presence.
- * Other header content stays verbatim, so a header-pinning fixture can keep
- * its complete tool schemas while every JSONL fixture omits the prompt text.
- * Lines without a system payload pass through byte-for-byte; the transform is
- * idempotent.
  * Replace the rendered prompt text of every `system/message` event with the
  * `{{system}}` token. The text block keeps its position and type, so the
  * fixture still shows one system node per prompt version; an empty `content`