Problem

The REPL had two critical usability issues:

1. History navigation broken: Up/down arrow keys did not recall previous commands
2. Double Ctrl+C timing issue: Users had to press Ctrl+C twice extremely quickly (within 300ms of the prompt appearing) to exit, making it nearly impossible to exit reliably

Root Cause

History Issue

The REPL spawned a new thread for each readline operation, and each thread created a new Editor instance. While the code attempted to maintain history by loading strings into each new editor, rustyline's built-in history navigation requires a persistent Editor instance with its own internal history state, not just a list of strings being copied around.

Double Ctrl+C Issue

The exit logic tracked last_was_interrupted as a boolean and checked if the readline had been active for less than 300ms. This meant users had to press Ctrl+C twice within 300ms of the prompt appearing, not within 300ms of each other - a timing that was nearly impossible to achieve reliably.

Solution

History Fix

• Wrap a single Editor instance in Arc<Mutex> and share it across threads
• This preserves rustyline's internal history state, enabling proper up/down arrow navigation
• History is now properly maintained across all readline operations

Double Ctrl+C Fix

• Track last_interrupt_time: Option<Instant> instead of a boolean flag
• Check if two Ctrl+C presses occur within 1 second of each other
• This gives users a reasonable, reliable window to exit the REPL

Changes

• File changed: cli/main.rs only
• Net change: -27 lines (simpler, more maintainable code)
• Preserved functionality: Event loop continues to tick during readline for async operations

Testing

✅ All 76 existing tests pass
✅ No clippy warnings
✅ Code properly formatted
✅ Manual testing confirms both issues are resolved

The threading approach remains necessary because readline() is a blocking call and we need the event loop to continue processing async operations. This fix preserves that architecture while fixing the usability issues.

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