feat(prompts): implement Phase 7 - Quality & Validation (FR-9, FR-10)

Add quality gate framework with schema validation (JSON Schema via
jsonschema library), pattern validation (regex-based), multi-gate
QualityValidator with SQLite persistence, HaltingPolicyEngine with
budget/iteration/improvement checks, and RefinementLoop for iterative
execute-validate-halt cycles.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

markitect/prompts/quality/gates/schema_gate.py

@@ -0,0 +1,123 @@
+"""
+Schema validation quality gate.
+
+Implements FR-9.1: Validate generated artifacts against JSON schemas.
+Uses the jsonschema library for validation.
+"""
+
+import json
+import uuid
+from typing import Any, Dict, Optional
+
+import jsonschema
+
+from markitect.prompts.quality.models import (
+    GateType,
+    QualityGate,
+    ValidationDiagnostic,
+    ValidationResult,
+    ValidationStatus,
+)
+
+
+class SchemaValidationGate(QualityGate):
+    """
+    Validates artifact content against a JSON schema.
+
+    Parses content as JSON and validates against the provided schema
+    using the jsonschema library.
+    """
+
+    def __init__(
+        self,
+        schema: Dict[str, Any],
+        gate_id: Optional[str] = None,
+        name: str = "schema",
+    ):
+        """
+        Initialize with a JSON schema.
+
+        Args:
+            schema: JSON Schema dictionary
+            gate_id: Optional gate identifier (auto-generated if not provided)
+            name: Human-readable gate name
+        """
+        super().__init__(
+            gate_id=gate_id or str(uuid.uuid4()),
+            name=name,
+            gate_type=GateType.SCHEMA,
+        )
+        self.schema = schema
+
+    def validate(self, content: str, artifact_id: str) -> ValidationResult:
+        """
+        Validate content against the JSON schema.
+
+        Parses the content as JSON, then validates against the schema.
+        Returns FAIL if content is not valid JSON or fails schema validation.
+
+        Args:
+            content: JSON content string to validate
+            artifact_id: ID of the artifact being validated
+
+        Returns:
+            ValidationResult with status and diagnostics
+        """
+        diagnostics = []
+
+        # Parse JSON
+        try:
+            data = json.loads(content)
+        except (json.JSONDecodeError, TypeError) as e:
+            diagnostics.append(
+                ValidationDiagnostic(
+                    code="INVALID_JSON",
+                    message=f"Content is not valid JSON: {e}",
+                    severity="error",
+                )
+            )
+            return ValidationResult.create(
+                gate_id=self.id,
+                gate_type=self.gate_type,
+                artifact_id=artifact_id,
+                status=ValidationStatus.FAIL,
+                score=0.0,
+                diagnostics=diagnostics,
+            )
+
+        # Validate against schema
+        validator = jsonschema.Draft7Validator(self.schema)
+        errors = list(validator.iter_errors(data))
+
+        if not errors:
+            return ValidationResult.create(
+                gate_id=self.id,
+                gate_type=self.gate_type,
+                artifact_id=artifact_id,
+                status=ValidationStatus.PASS,
+                score=1.0,
+                diagnostics=[],
+            )
+
+        for error in errors:
+            path = ".".join(str(p) for p in error.absolute_path) or "(root)"
+            diagnostics.append(
+                ValidationDiagnostic(
+                    code="SCHEMA_VIOLATION",
+                    message=f"At '{path}': {error.message}",
+                    severity="error",
+                )
+            )
+
+        # Score based on proportion of passing validations
+        total_checks = len(errors) + 1  # approximate
+        score = max(0.0, 1.0 - len(errors) / total_checks)
+
+        return ValidationResult.create(
+            gate_id=self.id,
+            gate_type=self.gate_type,
+            artifact_id=artifact_id,
+            status=ValidationStatus.FAIL,
+            score=score,
+            diagnostics=diagnostics,
+        )