feat: Implement BranchProtectionAssessor stub (fixes #86)

- Add BranchProtectionAssessor as Tier 4 stub assessor
- Returns not_applicable with explanation about GitHub API requirement
- Future implementation will verify:
  - Required status checks on main/master
  - Required approvals before merge
  - Force push prevention
  - Branch update requirements
- Provides clear path for future GitHub API integration
- Test: Correctly returns not_applicable for AgentReady

Author: jeremyeder

src/agentready/assessors/testing.py

@@ -642,3 +642,41 @@ def _create_remediation(self) -> Remediation:
                 ),
             ],
         )
+
+
+class BranchProtectionAssessor(BaseAssessor):
+    """Assesses branch protection rules on main/production branches.
+
+    Tier 4 Advanced (0.5% weight) - Requires GitHub API access to check
+    branch protection settings. This is a stub implementation that will
+    return not_applicable until GitHub API integration is implemented.
+    """
+
+    @property
+    def attribute_id(self) -> str:
+        return "branch_protection"
+
+    @property
+    def tier(self) -> int:
+        return 4  # Advanced
+
+    @property
+    def attribute(self) -> Attribute:
+        return Attribute(
+            id=self.attribute_id,
+            name="Branch Protection Rules",
+            category="Git & Version Control",
+            tier=self.tier,
+            description="Required status checks and review approvals before merging",
+            criteria="Branch protection enabled with status checks and required reviews",
+            default_weight=0.005,
+        )
+
+    def assess(self, repository: Repository) -> Finding:
+        """Stub implementation - requires GitHub API integration."""
+        return Finding.not_applicable(
+            self.attribute,
+            reason="Requires GitHub API integration for branch protection checks. "
+            "Future implementation will verify: required status checks, "
+            "required reviews, force push prevention, and branch update requirements.",
+        )

src/agentready/cli/main.py

@@ -41,6 +41,7 @@
     create_stub_assessors,
 )
 from ..assessors.testing import (
+    BranchProtectionAssessor,
     CICDPipelineVisibilityAssessor,
     PreCommitHooksAssessor,
     TestCoverageAssessor,
@@ -98,6 +99,8 @@ def create_all_assessors():
         SemanticNamingAssessor(),
         StructuredLoggingAssessor(),
         OpenAPISpecsAssessor(),
+        # Tier 4 Advanced (1 stub)
+        BranchProtectionAssessor(),
     ]
 
     # Add remaining stub assessors