docs: add Python style guide

[CRF1408]

Adds a style guide summarising Python coding conventions (PEP 8, Ruff, Black, type hints) for the agents codebase

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Adds concise Python conventions documentation for contributors.

• New docs/STYLE_GUIDE.md outlining use of PEP 8, ruff, black, type annotations, docstrings, and preference for small composable functions
• Guidance to apply these practices consistently across new modules and scripts

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