feat: Custom system prompt via .cora.yaml config

[ajianaz]

Summary

Allow users to override default system prompts via .cora.yaml config, supporting both inline strings and file references.

Accept Criteria

• Add review.system_prompt and review.system_prompt_file to CoraFile schema
• Add scan.system_prompt and scan.system_prompt_file to CoraFile schema
• Merge custom prompt: user prompt wraps/replaces default (configurable approach)
• File path resolution: relative to project root
• Graceful fallback: if custom prompt file not found, warn and use default
• cora config show displays custom prompt status
• Unit test: config parsing with custom prompt
• Unit test: file-not-found fallback

Scope

Config schema + loader + LLM engine prompt resolution

[ajianaz]

Closed in v0.1.6 release