Added fibonacci.py with fib(n) and main.

[opencode-agent]

Created fibonacci.py at the repository root.

Summary of changes:

• Added fibonacci(n) function with a clear docstring and input validation.
• Implemented an iterative (O(n)) approach to compute the nth Fibonacci number.
• Included a main block that prints the first 10 Fibonacci numbers:
  0, 1, 1, 2, 3, 5, 8, 13, 21, 34

Key implementation details:

• fibonacci(0) = 0, fibonacci(1) = 1
• Validates input: raises TypeError if n is not an int, raises ValueError if n < 0
• Main block outputs one number per line for i in range(10)

If you want a different indexing (e.g., 1-based), I can adjust the function and the main block accordingly.

Closes #690

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