Validate historySize and maxDiffSize when loading config

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Task summary

Add validation logic when loading config so invalid numeric values for historySize and maxDiffSize are rejected (or safely defaulted) with clear behavior.

Why this is a good first issue

The work is scoped to one module (src/config/store.ts) and can be completed with a small implementation plus targeted tests.

Suggested files or areas

• src/config/store.ts
• New/updated tests around config loading behavior

Acceptance criteria

• Define validation rules for historySize and maxDiffSize (e.g., must be positive integers).
• Ensure invalid values do not flow into runtime prompt generation.
• Add tests covering valid, missing, and invalid values.
• Document the chosen behavior in code comments or docs (defaulting vs throwing).

Extra context

loadConfig() currently applies defaults for missing fields but does not enforce numeric bounds for existing fields.