Pin request-header content in one snapshot scenario

Every session.jsonl fixture embedded the full composed system prompt and
complete tool-schema list in its request/header event (~8 KB on one line,
identical across the suite), so any prompt or tool-schema edit forced a
re-record or hand-edit of every fixture — see the dynamic-workflows PR for
the churn pattern this removes.

Now exactly one scenario (text-turn, flagged pinsHeader) commits and
compares that content verbatim; every other fixture stores and compares it
as {{system}}/{{tools}} tokens via the new pure scrubRequestHeaders
normalizer (applied to both compare sides and to record-mode writes, so a
re-record cannot reintroduce the content). request/header-delta payloads
are scrubbed the same way; config/reason stay verbatim — a model swap
SHOULD churn every fixture, a prompt edit should not. Replay is unaffected:
script derivation reads only assistant/chunk events.

Fixture meta-guards enforce the split: non-pinning fixtures must be fixed
points of the scrub, the pinning fixture must not be, and exactly one
scenario pins. Committed fixtures migrated through the same function.

Docs: pinned-header RFC (implemented/testing), base snapshot RFC + testing
policy + llm-replay module doc/README updated.

docs/rfc/INDEX.md

@@ -158,6 +158,7 @@ Generated by `pnpm run gen-rfc-index` from the RFC tree — never edit by hand;
 | [Per-session snapshot replay for nested agents](implemented/testing/2026-06-22-subagent-snapshot-replay.md) | 2026-06-22 |
 | [Hook snapshot matrix — end-to-end goldens for both bridges](implemented/testing/2026-07-04-hook-snapshot-matrix.md) | 2026-07-04 |
 | [Single-source the acp-agent replay config](implemented/testing/2026-07-04-single-source-acp-replay-config.md) | 2026-07-04 |
+| [Pin request-header content in one snapshot scenario](implemented/testing/2026-07-06-pin-request-header-content-in-one-scenario.md) | 2026-07-06 |
 
 ## Rejected