feat(code-review): add deterministic prefix golden harness (PLN-1229 Phase 0)

Build the deferred end-to-end prefix parity oracle for the run-prefix
refactor: a harness that walks the whole deterministic prefix (stages
01→19b) in-process against real pinned-date git fixtures, with the two
PLN-725 singletons stubbed and Gate A/B reproduced, snapshotting every
intermediate artifact for byte-level drift detection.

- prefix_golden_harness.py: fixture repo builder, hermetic env
  (HOME + gitconfig isolation), in-process stage executor via the real
  CLI parser, start.md angle-bracket token resolution, per-artifact
  normalization, PLN-725 singleton stubs, Gate A/B.
- 7 P0-B fixtures + committed goldens: standard, fast-path, hygiene-only,
  empty-diff, cache-hit, since-last-review, coverage-critic.
- test_prefix_golden.py: determinism oracle + golden compare + structural
  branch guards.
- P0-D: targeted branch tests close prefix cmd coverage gaps
  (cmd_cache_check / cmd_resolve_scope / cmd_auto_incremental /
  cmd_finalize_cache now 0 missing lines).

Full code-review suite: 1254 passed / 3 skipped. ruff + pyright clean.

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

Author: wongk

plugins/code-review/.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "code-review",
   "description": "Code review plugin",
-  "version": "3.1.0",
+  "version": "3.1.1",
   "author": {
     "name": "ClosedLoop",
     "email": "support@closedloop.ai"

plugins/code-review/tools/python/prefix_fixtures/golden_prefix_cache_hit/expected/agent_cached_bha.json

@@ -0,0 +1,19 @@
+{
+  "findings": [
+    {
+      "category": "Correctness",
+      "code_snippet": "C0 = 0",
+      "confidence": 0.8,
+      "emitted_at": "<TS>",
+      "explanation": "Prior review flagged an unnamed constant here.",
+      "file": "src/constants.py",
+      "finding_scope": "diff",
+      "issue": "Cached: magic constant should be named.",
+      "line": 1,
+      "recommendation": "Extract the literal into a named constant.",
+      "reviewer": "bha_p0",
+      "severity": "MEDIUM",
+      "source": "agent"
+    }
+  ]
+}

plugins/code-review/tools/python/prefix_fixtures/golden_prefix_cache_hit/expected/auto_incremental.json

@@ -0,0 +1,4 @@
+{
+  "diff_scope": null,
+  "review_mode_line": "Review mode: Auto incremental skipped: reason=no previous review, using full diff"
+}

plugins/code-review/tools/python/prefix_fixtures/golden_prefix_cache_hit/expected/available_reviewers.json

@@ -0,0 +1 @@
+[]

plugins/code-review/tools/python/prefix_fixtures/golden_prefix_cache_hit/expected/cache_config.json

@@ -0,0 +1,3 @@
+{
+  "cache_dir": "<HOME>/.claude/cr-cache-repo-fixture_repo"
+}

plugins/code-review/tools/python/prefix_fixtures/golden_prefix_cache_hit/expected/cache_result.json

@@ -0,0 +1,16 @@
+{
+  "cached_files": [
+    "src/constants.py"
+  ],
+  "stats": {
+    "cached": 1,
+    "hit_rate_pct": 50.0,
+    "total_files": 2,
+    "uncached": 1
+  },
+  "status_kind": "hits",
+  "status_message": "BHA Cache: 1/2 files cached (50.0% hit rate) -- 1 files skip BHA review",
+  "uncached_files": [
+    "src/app.py"
+  ]
+}

plugins/code-review/tools/python/prefix_fixtures/golden_prefix_cache_hit/expected/coverage.json

@@ -0,0 +1,101 @@
+{
+  "critic": {
+    "coverage_state": "<CR_DIR>/coverage.json",
+    "model": "sonnet",
+    "reason": "no-roster",
+    "status": "skipped"
+  },
+  "final": {
+    "best_effort": [],
+    "budget": {
+      "best_effort_count": 0,
+      "bha_partitions": 1,
+      "domain_critic_cap": 3,
+      "domain_critic_cap_fired": false,
+      "required_count": 4,
+      "total_cap": 20
+    },
+    "deferred_for_budget": [],
+    "deprecation_warnings": [],
+    "dropped_required": [],
+    "required": [
+      {
+        "reviewer": "bug_hunter_a",
+        "source": "core",
+        "trigger": {
+          "type": "always"
+        }
+      },
+      {
+        "reviewer": "bug_hunter_b",
+        "source": "core",
+        "trigger": {
+          "type": "always"
+        }
+      },
+      {
+        "reviewer": "unified_auditor",
+        "source": "core",
+        "trigger": {
+          "type": "always"
+        }
+      },
+      {
+        "reviewer": "test_quality",
+        "source": "core",
+        "trigger": {
+          "type": "always"
+        }
+      }
+    ]
+  },
+  "initial": {
+    "best_effort": [],
+    "generated_at": "<TS>",
+    "required": [
+      {
+        "reviewer": "bug_hunter_a",
+        "source": "core",
+        "trigger": {
+          "type": "always"
+        }
+      },
+      {
+        "reviewer": "bug_hunter_b",
+        "source": "core",
+        "trigger": {
+          "type": "always"
+        }
+      },
+      {
+        "reviewer": "unified_auditor",
+        "source": "core",
+        "trigger": {
+          "type": "always"
+        }
+      },
+      {
+        "reviewer": "test_quality",
+        "source": "core",
+        "trigger": {
+          "type": "always"
+        }
+      }
+    ],
+    "scope": "code-review",
+    "stats": {
+      "best_effort_count": 0,
+      "detected_change_classes": [],
+      "required_count": 4,
+      "rules_evaluated": 0,
+      "rules_matched": 0,
+      "signal_count": 1
+    },
+    "warnings": []
+  },
+  "verify": {
+    "checked_at": "<TS>",
+    "verdict": "PASS",
+    "violations": []
+  }
+}

plugins/code-review/tools/python/prefix_fixtures/golden_prefix_cache_hit/expected/coverage_gaps.json

@@ -0,0 +1,3 @@
+{
+  "findings": []
+}