feat(tui): add overlay compositing for ctx.ui.custom() (#558)

Adds overlay rendering capability to the TUI, enabling floating modal
components that render on top of existing content without clearing the screen.

- Add showOverlay(), hideOverlay(), hasOverlay() methods to TUI
- Implement ANSI-aware line compositing via extractSegments()
- Support overlay stack (multiple overlays, later on top)
- Add { overlay: true } option to ctx.ui.custom()
- Add overlay-test.ts example extension

Also fixes pre-existing bug where bash tool output cached visual lines
at fixed terminal width, causing crashes on terminal resize.

Co-authored-by: Nico Bailon <nico.bailon@gmail.com>

packages/coding-agent/src/core/extensions/types.ts

@@ -99,6 +99,7 @@ export interface ExtensionUIContext {
 			keybindings: KeybindingsManager,
 			done: (result: T) => void,
 		) => (Component & { dispose?(): void }) | Promise<Component & { dispose?(): void }>,
+		options?: { overlay?: boolean },
 	): Promise<T>;
 
 	/** Set the text in the core input editor. */