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/packages/hermes-ink/src/ink/selection.ts

@@ -799,6 +799,20 @@ export function hasSelection(s: SelectionState): boolean {
   return s.anchor !== null && s.focus !== null
 }
 
+/**
+ * Stable fingerprint of the user-visible selection state. Used by Ink
+ * to skip incrementing the mutation counter when notifySelectionChange()
+ * fires without an actual change to anchor/focus/isDragging — protects
+ * version-based subscribers (copy-on-select) from re-running for the
+ * same stable selection.
+ */
+export function selectionSignature(s: SelectionState): string {
+  const a = s.anchor ? `${s.anchor.row},${s.anchor.col}` : 'null'
+  const f = s.focus ? `${s.focus.row},${s.focus.col}` : 'null'
+
+  return `${a}|${f}|${s.isDragging ? 1 : 0}`
+}
+
 /**
  * Normalized selection bounds: start is always before end in reading order.
  * Returns null if no active selection.