[Feature]: add configurable system prompts feature

[OreoYang]

Problem Statement

Since IvorySQL is a PostgreSQL-based database with Oracle compatibility features, modifications to system prompts are required.

Proposed Solution

• Automatic fallback to built-in defaults when not configured

• Option 1: Inline String (Short Prompts)

~/.pg_ai.config
[prompts]
system_prompt = "You are a PostgreSQL expert for financial data. Always include index recommendations."
explain_system_prompt = "Focus on materialized view usage and query optimization."

• Option 2: File Path (Long Prompts - Recommended)

~/.pg_ai.config
[prompts]
system_prompt = /home/user/.pg_ai.prompts
explain_system_prompt = /home/user/.pg_ai.explain.prompts

Alternatives Considered

No response

Feature Category

AI/ML improvements

Use Case

• add test suite PromptsTest tests/unit/test_prompts.cpp

Example Usage

Priority

Critical - Blocking my work

Contribution

• I would be willing to submit a PR for this feature
• I would be willing to help test this feature

Additional Context

No response

[OreoYang]

PR #86