🎨 Palette: Improve CLI error handling for invalid inputs

[EiJackGH]

• 💡 What: Added input validation for days, initial_cash, initial_price, and volatility in bitcoin_trading_simulation.py.
• 🎯 Why: To prevent confusing errors or crashes when users provide negative or zero values.
• 📸 Before/After: Before, negative values caused silent failures or tracebacks. Now, they produce friendly red error messages.
• ♿ Accessibility: Improved error clarity reduces cognitive load.
• Added test_cli_args.py to verify the new behavior.
• Updated .Jules/palette.md with learnings.

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PR created automatically by Jules for task 1214904420847840075 started by @EiJackGH

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