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

cli-config.yaml.example

@@ -927,7 +927,7 @@ display:
   #     agent_name: "My Agent"               # Banner title and branding
   #     welcome: "Welcome message"           # Shown at CLI startup
   #     response_label: " ⚔ Agent "         # Response box header label
-  #     prompt_symbol: "⚔ ❯ "              # Prompt symbol
+  #     prompt_symbol: "⚔"                  # Prompt symbol (bare token; renderers add trailing space)
   #   tool_prefix: "╎"                       # Tool output line prefix (default: ┊)
   #
   skin: default