Document agent feedback field limits (#99415)
## Summary
Document the schema v5 field-length limits enforced by the agent
feedback review form so agents can prepare valid report links instead of
receiving a generic decode failure.
Add a focused test that keeps the protocol aligned with the form limits.
## Verification
- `pnpm exec jest
packages/next/src/cli/internal/agent-feedback-instructions.test.ts
--runInBand`
- Prettier and ESLint on the changed files
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