feat: added scrollback rebuild controls and prewalk status-line visibility

- Added `tui.scrollbackRebuild` configuration with interactive startup/controller wiring to apply `setScrollbackRebuild`.
- Exposed prewalk session state in `SegmentContext` and rendered a dedicated prewalk segment/icon in the status line.
- Added divergence-aware TUI full-paint logic that enables scrollback erase-and-replay rebuilds for non-multiplexer divergence cases.
- Updated rendering and streaming tests to verify rebuild behavior (`3J`) and eliminate stale marker expectations under drift scenarios.

packages/coding-agent/test/status-line-model.test.ts

@@ -22,6 +22,7 @@ function createModelContext(advisorActive: boolean): SegmentContext {
 		options: {},
 		planMode: null,
 		loopMode: null,
+		prewalk: null,
 		goalMode: null,
 		vibeMode: null,
 		collab: null,