docs: add coding standards for agent reference

docs/coding-standards.md

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+# Coding Standards for SNS Network Solutions
+
+## Python
+- Use type hints on all function signatures
+- Use dataclasses for data containers (not plain dicts)
+- Use pathlib.Path (never os.path)
+- Use httpx (not requests) for HTTP calls
+- Always include docstrings on public functions
+- Error handling: use specific exceptions, never bare except
+
+## File Structure
+- One module per concern
+- Tests in tests/ directory, mirroring src/ structure
+- Config in config/ as YAML
+
+## Naming
+- snake_case for functions and variables
+- PascalCase for classes
+- UPPER_CASE for constants
+- Prefix private functions with underscore
+
+## Patterns
+- Dependency injection over global state
+- Async by default for I/O operations
+- Return early on error (guard clauses)
+