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__/terminalParity.test.ts

@@ -28,6 +28,12 @@ describe('terminalParityHints', () => {
   it('suppresses IDE setup hint when keybindings are already configured', async () => {
     const readFile = vi.fn().mockResolvedValue(
       JSON.stringify([
+        {
+          key: 'cmd+c',
+          command: 'workbench.action.terminal.sendSequence',
+          when: 'terminalFocus && terminalTextSelected',
+          args: { text: '\u001b[99;13u' }
+        },
         {
           key: 'shift+enter',
           command: 'workbench.action.terminal.sendSequence',