fix: implement stage_complete tool for LLM-driven stage completion

- Add STAGE_COMPLETE_TOOL constant and meta-tool handler in executeStage
  loop: when LLM calls stage_complete, break out of tool dispatch loop
  and proceed to normal validation/transition
- Inject stage_complete tool definition in every inference request so
  the LLM always has the option to signal stage completion
- Replace expand-all diff with scrollable diff window in approval
  surface (ctrl+x toggles, up/down scrolls, shows line range)

core/kernel/src/main/kotlin/com/correx/core/kernel/orchestration/SessionOrchestrator.kt

@@ -92,6 +92,7 @@ import kotlinx.coroutines.withContext
 import kotlinx.coroutines.withTimeout
 import kotlinx.datetime.Clock
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonObject
 import org.slf4j.LoggerFactory
 import java.util.*
 import java.util.concurrent.*
@@ -99,6 +100,7 @@ import java.util.concurrent.atomic.*
 import kotlin.coroutines.cancellation.CancellationException
 
 private const val MAX_TOOL_ROUNDS = 10
+private const val STAGE_COMPLETE_TOOL = "stage_complete"
 
 @SuppressWarnings(
     "ForbiddenComment",
@@ -258,6 +260,12 @@ abstract class SessionOrchestrator(
             inferenceResult.response.finishReason is FinishReason.ToolCall &&
             toolRounds < MAX_TOOL_ROUNDS
         ) {
+            // stage_complete is a meta-tool: the LLM calls it to signal the stage's goal is met.
+            // Skip tool dispatch — the loop exits and normal validation/transition follows
+            // (emitToolArtifacts + verifyProduces run on the success path after the loop).
+            if (inferenceResult.response.toolCalls.any { it.function.name == STAGE_COMPLETE_TOOL }) {
+                break
+            }
             val toolEntries = dispatchToolCalls(sessionId, stageId,
                 inferenceResult.response.toolCalls, stageConfig)
             val fatalEntry = toolEntries.firstOrNull { it.content.startsWith("FATAL:") }
@@ -670,7 +678,14 @@ abstract class SessionOrchestrator(
                             parameters = tool.parametersSchema,
                         ),
                     )
-                },
+                } + ToolDefinition(
+                    function = ToolFunction(
+                        name = STAGE_COMPLETE_TOOL,
+                        description = "Call this tool when the stage's goal is fully met and no further tool calls are needed. " +
+                                "The orchestrator will proceed to the next stage.",
+                        parameters = JsonObject(emptyMap()),
+                    ),
+                ),
         )
 
         // TODO: capture rendered prompt; currently contextpack.toString()