[codex] Add deslopify review mode

[mbelinky]

Summary

Adds a narrow review --mode deslopify lane for slop-driven simplification and performance findings. The default review path remains broad for correctness, security, API contract, and other risk categories.

The new mode:

• passes deslopify-specific instructions into the review prompt
• restricts stored provider findings to maintainability and performance
• rejects unsupported review modes such as simplify
• documents the narrow rubric and CLI usage

Validation

• pnpm exec oxfmt --write README.md docs/code-review.md src/app.ts src/cli.ts src/prompt.ts src/workflow.test.ts
• pnpm test src/workflow.test.ts
• pnpm typecheck
• pnpm build
• git diff --check
• local CLI smoke: review --mode simplify --dry-run rejects the mode
• local CLI smoke: review --mode deslopify --limit 1 --dry-run reports mode: deslopify

Notes

This intentionally does not add report clustering or deslopify-specific ranking; that should be a follow-up PR.

[steipete]

This stacked change landed through #56 in eabfff0. Closing this lower PR to keep the queue clean.

Thanks @mbelinky.