🧹 Extract REPL metacommand handling to reduce nesting

[Tcode-Motion]

🎯 What: Extracted the deeply nested match cmd block inside the REPL's execute function into a new handle_metacommand helper function.
💡 Why: The original execute function was quite long and deeply nested (a match inside an if/else inside a match inside a loop), which hurt readability and maintainability. Abstracting the metacommand processing flattens the loop and simplifies the main REPL logic.
✅ Verification: Verified by checking types locally (*interpreter dereferencing), confirming no API changes, running package tests (cargo test -p techscript_cli), and running full workspace tests (cargo test --workspace), all of which passed perfectly. Requested and passed internal code review.
✨ Result: The execute function is significantly shorter and more readable, focusing on the REPL lifecycle rather than individual command implementations.

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PR created automatically by Jules for task 15863708305393372875 started by @Tcode-Motion

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