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)

apps/tui/src/main/kotlin/com/correx/apps/tui/components/ApprovalSurface.kt

@@ -27,6 +27,7 @@ fun approvalSurfaceWidget(state: TuiState): Paragraph {
     val pendingDecision = state.pendingDecision
 
     val diffExpanded = state.diffExpanded
+    val scrollOffset = state.diffScrollOffset
 
     val lines = buildList<Line> {
         if (approval == null) {
@@ -45,8 +46,11 @@ fun approvalSurfaceWidget(state: TuiState): Paragraph {
             if (preview != null) {
                 val previewLines = preview.lines()
                 if (previewLines.isNotEmpty()) {
+                    val totalLines = previewLines.size
                     val visibleLines = if (diffExpanded) {
-                        previewLines
+                        val from = scrollOffset.coerceIn(0, maxOf(0, totalLines - 1))
+                        val to = minOf(from + PREVIEW_MAX_LINES, totalLines)
+                        previewLines.subList(from, to)
                     } else {
                         previewLines.take(PREVIEW_MAX_LINES)
                     }
@@ -69,12 +73,13 @@ fun approvalSurfaceWidget(state: TuiState): Paragraph {
                             add(Line.from(Span.raw("  $line")))
                         }
                     }
-                    if (!diffExpanded && previewLines.size > PREVIEW_MAX_LINES) {
-                        val remaining = previewLines.size - PREVIEW_MAX_LINES
-                        add(Line.from(Span.styled("  (preview truncated, $remaining more lines — ctrl+x to expand)", dimStyle)))
+                    if (!diffExpanded && totalLines > PREVIEW_MAX_LINES) {
+                        val remaining = totalLines - PREVIEW_MAX_LINES
+                        add(Line.from(Span.styled("  (preview truncated, $remaining more lines — ctrl+x to expand, \u2191\u2193 to scroll)", dimStyle)))
                     }
                     if (diffExpanded) {
-                        add(Line.from(Span.styled("  (full diff — ctrl+x to collapse)", dimStyle)))
+                        val showingTo = minOf(scrollOffset + PREVIEW_MAX_LINES, totalLines)
+                        add(Line.from(Span.styled("  lines ${scrollOffset + 1}\u2013$showingTo of $totalLines — \u2191\u2193 scroll, ctrl+x collapse", dimStyle)))
                     }
                 } else {
                     add(Line.from(Span.styled("  no preview available", dimStyle)))

apps/tui/src/main/kotlin/com/correx/apps/tui/input/KeyResolver.kt

@@ -72,6 +72,8 @@ object KeyResolver {
                 KeyEvent.Enter -> Action.SubmitApprovalDecision
                 KeyEvent.Escape -> Action.CancelInput
                 KeyEvent.Cancel -> Action.CancelInput
+                KeyEvent.NavUp -> Action.NavigateUp
+                KeyEvent.NavDown -> Action.NavigateDown
                 KeyEvent.ReturnToSessionList -> Action.ReturnToSessionList
                 KeyEvent.ToggleEventStrip -> Action.ToggleEventStrip
                 KeyEvent.ToggleDiffExpand -> Action.ToggleDiffExpand

apps/tui/src/main/kotlin/com/correx/apps/tui/reducer/RootReducer.kt

@@ -118,7 +118,10 @@ object RootReducer {
                 else withBgReset.copy(eventStripVisible = !withBgReset.eventStripVisible)
             }
 
-            is Action.ToggleDiffExpand -> withBgReset.copy(diffExpanded = !withBgReset.diffExpanded)
+            is Action.ToggleDiffExpand -> withBgReset.copy(
+                diffExpanded = !withBgReset.diffExpanded,
+                diffScrollOffset = 0,
+            )
 
             // Input history append on SubmitInput in IN_SESSION + ROUTER (ARCH-HISTORY-1),
             // or enter a selected session from the list in IDLE.
@@ -188,9 +191,11 @@ object RootReducer {
                 }
             }
 
-            // Input history navigation on NavigateUp in IN_SESSION + ROUTER (ARCH-HISTORY-2/3).
+            // Diff scrolling in APPROVAL mode when expanded.
             is Action.NavigateUp -> {
-                if (withBgReset.displayState == DisplayState.IN_SESSION && prevInputMode == InputMode.ROUTER) {
+                if (withBgReset.displayState == DisplayState.APPROVAL && withBgReset.diffExpanded) {
+                    withBgReset.copy(diffScrollOffset = maxOf(0, withBgReset.diffScrollOffset - 1))
+                } else if (withBgReset.displayState == DisplayState.IN_SESSION && prevInputMode == InputMode.ROUTER) {
                     val selectedId = withBgReset.sessions.selectedId
                     if (selectedId != null) {
                         val history = withBgReset.inputHistory[selectedId] ?: emptyList()
@@ -227,9 +232,11 @@ object RootReducer {
                 }
             }
 
-            // Input history navigation on NavigateDown in IN_SESSION + ROUTER (ARCH-HISTORY-2/3).
+            // Diff scrolling in APPROVAL mode when expanded.
             is Action.NavigateDown -> {
-                if (withBgReset.displayState == DisplayState.IN_SESSION && prevInputMode == InputMode.ROUTER) {
+                if (withBgReset.displayState == DisplayState.APPROVAL && withBgReset.diffExpanded) {
+                    withBgReset.copy(diffScrollOffset = withBgReset.diffScrollOffset + 1)
+                } else if (withBgReset.displayState == DisplayState.IN_SESSION && prevInputMode == InputMode.ROUTER) {
                     val selectedId = withBgReset.sessions.selectedId
                     if (selectedId != null) {
                         val history = withBgReset.inputHistory[selectedId] ?: emptyList()

apps/tui/src/main/kotlin/com/correx/apps/tui/state/TuiState.kt

@@ -48,6 +48,7 @@ data class TuiState(
     val routerConnected: Boolean = false,
     val routerMessages: Map<String, List<String>> = emptyMap(),
     val diffExpanded: Boolean = false,
+    val diffScrollOffset: Int = 0,
     val chatMode: ChatMode = ChatMode.CHAT,
     /**
      * True from connect until SnapshotComplete observed. Reset to true on Disconnected.