fix(tui): restore macOS copy behavior and theme polish (#17131)

This PR groups the TUI fixes that restore macOS Terminal usability and clean up the theme/composer regressions:

- copy transcript selections on macOS drag-release so Terminal.app users can copy while mouse tracking is enabled
- copy composer selections on macOS drag-release; composer selection is internal to TextInput and does not use the global Ink selection bus
- keep IDE Cmd+C forwarding setup macOS-only, and make keybinding conflict checks respect simple when-clause overlap/negation
- force truecolor before chalk initializes (unless NO_COLOR / FORCE_COLOR / HERMES_TUI_TRUECOLOR opt-outs apply) so the default banner keeps its gold/amber/bronze gradient in Terminal.app
- move TUI surfaces onto semantic theme tokens and preserve skin prompt symbols as bare tokens with renderer-owned spacing
- render focused placeholders as dim hint text in TTY mode instead of inverse/selected-looking synthetic cursor text

ui-tui/src/__tests__/textInputWrap.test.ts

@@ -44,6 +44,7 @@ describe('input metrics helpers', () => {
 
   it('reserves gutters on wide panes without starving narrow composer width', () => {
     expect(stableComposerColumns(100, 3)).toBe(93)
+    expect(stableComposerColumns(100, 5)).toBe(91)
     expect(stableComposerColumns(10, 3)).toBe(5)
     expect(stableComposerColumns(6, 3)).toBe(1)
   })