Lightpanda agent REPL renders styled terminal markdown

Making terminal-based AI outputs human-friendly is a crucial UX touch that sets developer-focused tools apart, and implementing a custom Zig-based markdown-to-ANSI parser showcases Lightpanda's dedication to zero-dependency lightweight performance.

* Enhances interactive agent debugging by translating markdown syntax like bold, italic, and strikeout into proper ANSI sequences.

* Includes sophisticated OSC 8 clickable hyperlink support that falls back gracefully on older terminals by appending a dim URL.

* Integrates parent style tracking via a stack-threaded chain to maintain visual formatting consistency in deeply nested markdown.

* Preserves automated workflows by gating rendering to real interactive TTY sessions, avoiding breaking downstream log parsers.

* Resolves readability concerns by switching from 256-ramp gray to standard SGR 2 dim styling for de-emphasized elements.