Add interactive debugger with frame locals, eval-in-frame, and inspect

Implement a full-featured debugger TUI that supports:
- Frame locals inspection (Enter on backtrace frame fetches locals)
- Eval-in-frame (e key evaluates expressions in frame context)
- Inspect frame variables (Enter on a local launches inspector)
- Interactive restart values (USE-VALUE/STORE-VALUE prompt for input)
- Post-mortem debugging via ,debug command
- Theme-safe styling using bold/dim instead of hardcoded colors

Adds slime-eval-in-thread to slynk-client for synchronous eval on
debug threads, and hooks for :debug/:debug-return events.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>