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__/streamingMarkdown.test.ts

@@ -74,6 +74,6 @@ describe('streaming theme assumption', () => {
     // Sanity that the theme we pass doesn't change shape. Component import
     // already happens above — this is a smoke test that the module graph
     // for streamingMarkdown wires up without cycles.
-    expect(DEFAULT_THEME.color.amber).toBeTruthy()
+    expect(DEFAULT_THEME.color.accent).toBeTruthy()
   })
 })