feat(review): add configurable approval prompts

[oorestisime]

Summary

• add prompts.review.approved and prompts.review.runtimes.<runtime>.approved to ~/.plannotator/config.json
• route code review approval messages through a shared prompt lookup in the hook, OpenCode, and Pi integrations while keeping the current message as the default fallback
• document the new config surface and cover prompt lookup and merge behavior with shared tests

Testing

• bun test packages/shared/prompts.test.ts packages/shared/feedback-templates.test.ts

Open Questions

• review approval prompts are currently keyed by generic/runtime override, not by review flow. Should this become more granular for flows like pull-request vs local diff modes (staged, uncommitted, last-commit, etc.) before prompt customization expands further?
• runtime identifiers already exist in packages/shared/agents.ts, but the prompt config duplicates that union in packages/shared/config.ts to keep the vendored Pi config self-contained. Is there a better centralization path here?
• nested prompt config is merged manually inside packages/shared/config.ts. If prompt customization grows beyond this first review-approval case, should this move toward a more structured typed config layer?

Fixes: #558

[backnotprop]

in review