docs(code-review): changelog + README for prefix golden harness

Add the v3.1.1 CHANGELOG entry and list the new prefix harness / test /
fixtures in the README repository-layout section.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: wongk

CHANGELOG.md

@@ -4,6 +4,12 @@ All notable changes to the claude-plugins project will be documented in this fil
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/). Entries are listed newest-first; each plugin section is treated as released when merged to `main`.
 
+### code-review v3.1.1
+
+#### Added
+- Deterministic golden-fixture harness for the review pipeline's front half: it walks the whole deterministic prefix (setup through reviewer-spawn-spec derivation) in-process against real, pinned-date git fixtures — with the signal-extraction and coverage-critic singleton agents stubbed and the hygiene-only exit and routing/fast-path branches reproduced — and snapshots every intermediate artifact for byte-level regression detection. Ships seven fixtures (standard/partitioned, fast-path, hygiene-only, empty-diff, cache-hit, since-last-review, coverage-critic) with committed golden snapshots and a cross-run determinism check.
+- Targeted branch tests for the `cache-check`, `resolve-scope`, `auto-incremental`, and `finalize-cache` helpers covering their previously-untested degradation and mode/scope paths (missing diff data, unreadable setup, forced/auto incremental rebase and same-head fallbacks, and the GitHub/global/PR-scoped cache directories).
+
 ### code-review v3.1.0
 
 #### Added

plugins/code-review/README.md

@@ -37,6 +37,9 @@ plugins/code-review/
     python/golden_fixture_harness.py     Golden fixture harness: replays canonical inputs through helper subcommands and diffs against expected envelopes (PLN-719 Phase 8)
     python/test_golden_fixtures.py       Pytest driver that runs every fixture under tools/python/fixtures/
     python/fixtures/<name>/              Per-fixture directory (config.yaml + inputs/ + expected/); 3 full scenarios + 6 README-stubs for future coverage
+    python/prefix_golden_harness.py      Prefix golden harness: walks the deterministic prefix (setup→spawn-spec) in-process against real git fixtures and snapshots every intermediate artifact (PLN-1229 Phase 0)
+    python/test_prefix_golden.py         Pytest driver for the prefix harness: determinism oracle + golden diff across the prefix_fixtures/ matrix
+    python/prefix_fixtures/<name>/       Per-fixture directory (expected/ golden snapshots); 7 branch scenarios (standard, fast-path, hygiene-only, empty-diff, cache-hit, since-last-review, coverage-critic)
 ```
 
 ### Foundation (PLN-719, schema_version 1)