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/testing.md

@@ -7,7 +7,7 @@ How this repo tests, tier by tier, and the rules that keep a green suite meaning
 - **Unit** (`pnpm run test`): vitest over `packages|examples/*/tests/**/*.spec.ts`, colocated with what they test. Every registry gets an HMR-safety test (dispose the contributing fiber, assert cleanup). Excessive tests are welcome — err toward covering edge cases, error paths, event ordering, and concurrency races; review findings get regression tests (see `packages/core/agent-loop/tests/review-fixes.spec.ts`).
 - **Coverage gate** (`pnpm run test:coverage`): the gating run, per-file 100% on `packages/*/*/src`. An uncovered line is often dead code the gate is correctly flagging for deletion, not a missing test to bolt on. Line coverage is necessary, never sufficient — it proves lines ran, not that the feature works as shipped.
 - **Real-API e2e** (`pnpm run test:e2e`): with-key tests against live provider APIs — the DeepSeek model plus provider-specific smokes that gate on their own keys (`EXA_API_KEY`, `PERPLEXITY_API_KEY`, …); each suite self-skips without its key so keyless CI stays green ([real-API e2e RFC](rfc/implemented/testing/2026-06-19-real-api-e2e-ci.md)).
-- **Snapshot** (`pnpm run test:snapshot`): boots the real example subprocess, replays a recorded session keyless, diffs normalized stdout + the re-persisted log against committed goldens ([snapshot RFC](rfc/implemented/testing/2026-06-19-acp-snapshot-tests.md)). Re-record with `pnpm run test:snapshot:record`; reviewing the golden diff is part of the review.
+- **Snapshot** (`pnpm run test:snapshot`): boots the real example subprocess, replays a recorded session keyless, diffs normalized stdout + the re-persisted log against committed goldens ([snapshot RFC](rfc/implemented/testing/2026-06-19-acp-snapshot-tests.md)). Re-record with `pnpm run test:snapshot:record`; reviewing the golden diff is part of the review. System-prompt/tool-schema content is pinned by ONE scenario (`text-turn`) and tokenized in every other fixture, so a prompt or schema edit churns one committed line ([pinned-header RFC](rfc/implemented/testing/2026-07-06-pin-request-header-content-in-one-scenario.md)).
 
 ## The with-key policy: inference is cheap here