fix(tui): reduce unnecessary full redraws for better performance

- Remove height change detection (only width changes trigger full redraw)
- Change clearOnShrink default to false (use PI_CLEAR_ON_SHRINK=1 to enable)
- Fix viewport check to use previousLines.length instead of maxLinesRendered
  (prevents false positive redraws when appending lines after content shrunk)
- Add clearOnShrink setting to /settings in coding-agent
- Remove line truncation in custom message component (always show full content)

packages/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- **Clear on shrink setting**: New `terminal.clearOnShrink` setting (and `/settings` toggle) controls whether empty rows are cleared when content shrinks. Disabled by default to reduce flicker. Enable via settings or `PI_CLEAR_ON_SHRINK=1` env var.
+
 ## [0.51.0] - 2026-02-01
 
 ### Breaking Changes