Add a theme token for hyperlink/link text styling

Claude Code emits clickable links — PR/issue references (#1031), URLs, file paths — that open correctly on click. But there's no visual signal that the text is a link before you click it.

In fullscreen rendering mode, this is worse than it sounds: Claude Code captures mouse events itself, which means the terminal's native OSC 8 hover/underline affordance never fires. Background/agent-view sessions always run fullscreen and can't opt out (/tui default and CLAUDE_CODE_DISABLE_ALTERNATE_SCREEN are documented as not applying there), so there's no config path back to terminal-native hyperlink hints in those sessions.

I checked the current theme token reference (/docs/en/terminal-config#color-token-reference) — there's no token for link/hyperlink text among the ~35 documented tokens or the overridable set. Tokens exist for status colors, diffs, fullscreen backgrounds, subagent colors, etc., but nothing that marks "this text is a clickable reference."

Request: add a link (or hyperlink) color token to the theme system, with a default style that's visually distinct from plain text — color, underline, or both — consistent across the dark/light/ansi presets. Since fullscreen mode has no hover state to fall back on, a persistent style (not interaction-dependent) would matter more here than it does in terminals with native OSC 8 support.

Related: #48652 (OSC 8 in TUI output) and #13008 (OSC 8 for file paths) — this is specifically about the missing visual affordance, separate from the click-handling those cover.

Terminal: Ghostty, macOS
Claude Code renderer: fullscreen (including agent-view/background sessions)