feat(api): switch func calling to tools

[appleboy]

• Add imports for "fmt" and "log" in commit.go
• Refactor message output to use a variable in commit.go
• Update error handling for completion response to check for both errors and the number of choices in commit.go
• Simplify the handling of summary prefix by removing conditional checks and replacing them with a single error check and assignment in commit.go
• Change CreateFunctionCall to accept a single FunctionDefinition instead of a variadic slice in openai.go
• Remove hardcoded system message content in openai.go
• Replace Functions field with Tools in CreateFunctionCall to use the new Tool struct in openai.go
• Remove specific function call strings from the allow list in openai.go and replace with a single entry

fix #156