feat: add MissingToolRule semantic validation

Flags when a workflow stage's allowedTools references a tool name that
is not registered in the tool registry (config drift), surfaced as a
non-blocking WARNING.

- Add ValidationContext.availableTools (nullable; null = registry
  unavailable, skip the check)
- MissingToolRule emits one MISSING_TOOL warning per (stage, tool) pair,
  deterministically ordered
- Populate availableTools from the ToolRegistry at the orchestrator
  construction site; wire the rule into the prod SemanticValidator

core/validation/src/main/kotlin/com/correx/core/validation/model/ValidationContext.kt

@@ -10,4 +10,5 @@ data class ValidationContext(
     val detectedCycles: List<DetectedCycle> = emptyList(),
     val cyclePolicies: Set<CyclePolicyBinding> = emptySet(),
     val sessionState: SessionState? = null,
+    val availableTools: Set<String>? = null,
 )

core/validation/src/main/kotlin/com/correx/core/validation/semantic/rules/MissingToolRule.kt

@@ -0,0 +1,34 @@
+package com.correx.core.validation.semantic.rules
+
+import com.correx.core.validation.model.ValidationContext
+import com.correx.core.validation.model.ValidationIssue
+import com.correx.core.validation.model.ValidationLocation
+import com.correx.core.validation.model.ValidationSeverity
+import com.correx.core.validation.semantic.SemanticRule
+
+class MissingToolRule : SemanticRule {
+
+    override fun validate(context: ValidationContext): List<ValidationIssue> {
+        val available = context.availableTools ?: return emptyList()
+
+        val issues = mutableListOf<ValidationIssue>()
+
+        for (stageId in context.graph.stageIds.sortedBy { it.value }) {
+            val stageConfig = context.graph.stages[stageId] ?: continue
+            for (toolName in stageConfig.allowedTools.sorted()) {
+                if (toolName !in available) {
+                    issues.add(
+                        ValidationIssue(
+                            code = "MISSING_TOOL",
+                            message = "Stage '${stageId.value}' allows unregistered tool '$toolName'",
+                            severity = ValidationSeverity.WARNING,
+                            location = ValidationLocation.Graph(stageId = stageId)
+                        )
+                    )
+                }
+            }
+        }
+
+        return issues
+    }
+}