feat: shell execution correctness + artifact_field_equals condition

- Add  field to ProcessResultArtifact ("success"/"failed")
- Materialize ToolResult.Failure as a failed ProcessResult artifact
  in the orchestrator, with artifact lifecycle events emitted
- Non-recoverable ToolResult.Failure now fails the stage ("FATAL:")
  instead of being silently fed back as LLM context
- Fix enterStage to return Continue on non-retryable failures,
  enabling back-edge transitions (fix/retry loops)
- Add ArtifactFieldEquals transition condition with flat and
  nested dot-notation field access + FieldOperator enum
  (eq, neq, gt, gte, lt, lte)
- Wire artifact_field_equals through ConditionFactory and
  TomlWorkflowLoader (condition_field, condition_operator)
- Cache ProcessResult artifact content for evaluation context
- Add 16 tests covering all condition operators and error cases
- DefaultTransitionResolver catches condition evaluation errors
  and returns Blocked instead of crashing

core/artifacts/src/main/kotlin/com/correx/core/artifacts/kind/ProcessResultArtifact.kt

@@ -4,6 +4,7 @@ import kotlinx.serialization.Serializable
 
 @Serializable
 data class ProcessResultArtifact(
+    val status: String = "success",
     val command: String,
     val exitCode: Int,
     val stdout: String,

core/artifacts/src/main/kotlin/com/correx/core/artifacts/kind/ProcessResultKind.kt

@@ -10,13 +10,14 @@ object ProcessResultKind : ArtifactKind {
     override fun deriveJsonSchema(): JsonSchema = JsonSchema(
         type = "object",
         properties = mapOf(
+            "status" to JsonSchemaProperty(type = "string", description = "\"success\" or \"failed\""),
             "command" to JsonSchemaProperty(type = "string"),
             "exitCode" to JsonSchemaProperty(type = "integer"),
             "stdout" to JsonSchemaProperty(type = "string"),
             "stderr" to JsonSchemaProperty(type = "string"),
             "durationMs" to JsonSchemaProperty(type = "integer"),
         ),
-        required = listOf("command", "exitCode", "stdout", "stderr", "durationMs"),
+        required = listOf("status", "command", "exitCode", "stdout", "stderr", "durationMs"),
         additionalProperties = false,
     )
 }