style: use type-only imports

[github-actions]

Closes #305

Applied changes based on Issue #305.

I’ve converted all purely type-based imports to import type while leaving runtime imports intact:

• src/api/openai.ts: split out ClientOptions and ActionConfig as type-only imports.
• src/client/codex.ts: prefixed ActionConfig import with type.
• src/github/action.ts: prefixed ActionConfig and ProcessedEvent imports with type.
• src/github/event.ts: split AgentEvent out as a type-only import and prefixed ActionConfig with type, leaving runtime helpers.
• src/security/security.ts: prefixed ActionConfig and Octokit imports with type.

These changes ensure TS will strip them from the emitted JS while preserving runtime behavior.