Add code_review phase with approval gate, reviewer≠implementer enforcement, and structured CODE_REVIEW.md

- Insert code_review phase between implement and bug_find
- Approval gate: code_review:awaiting_approval → code_review:approved
- Read-only phase — no edits, no fixes, no returning to implement
- Reviewer≠implementer: .state.implementer tracking + --claim enforcement
- Structured CODE_REVIEW.md: spec compliance, design conformance, quality
  scorecard, items found (severity/category/location/resolution), test coverage
- Updated status.py (10 data structures), dashboard (4 files), prompts (3 files),
  agent routing, tests (6 new test classes, 19 new tests)

.agent.md

@@ -11,6 +11,7 @@ IF task type = research     → load prompts/research.md + .rules.md
 IF task type = design       → load prompts/design.md + SPEC.md
 IF task type = test_design  → load prompts/test_design.md + SPEC.md + DESIGN.md
 IF task type = implement    → load prompts/implement.md + SPEC.md + DESIGN.md + TEST_PLAN.md + CONTRACT.md
+IF task type = code_review   → load prompts/code_review.md + SPEC.md + DESIGN.md + IMPLEMENTATION.md
 IF task type = bug_find     → load prompts/bug_finder.md + SPEC.md + code
 IF task type = adversarial_bug_find → load prompts/adversarial_bug_find.md + SPEC.md + code
 IF task type = referee      → load prompts/referee.md + SPEC.md + BUG_REPORT.md + ADVERSARIAL_BUG_REPORT.md + DOC_REVIEW.md
@@ -45,6 +46,9 @@ Agents:
     phases: [research, decomposition, design, test_design]
   - id: implementer
     phases: [implement]
+  - id: code-reviewer
+    phases: [code_review]
+    role: reviewer
   - id: bug-hunter
     phases: [bug_find, adversarial_bug_find]
   - id: referee