Add a PR review size rule for small, reviewable changes

[jabelic]

Goal

Document a repository rule that keeps each Pull Request around 300 changed lines, excluding lock file changes, so reviews stay small and manageable.

Background

Across Jabelic-Works repositories, we want to prefer small, reviewable Pull Requests.
The same expectation should apply to the initial Pull Request, and larger work should be split instead of bundled together.
This repository is not available locally right now, so we are tracking the change as a GitHub Issue.

Scope

• Add .cursor/rules/pr-review-size.mdc
• Make the rule alwaysApply: true
• State that each Pull Request should usually stay around 300 changed lines excluding lock file changes
• Apply the same size target to the initial Pull Request
• Instruct contributors to split setup, scaffolding, and follow-up work into separate Pull Requests when the change would exceed the target

Out of Scope

• Adding CI enforcement for diff size
• Revisiting the full contributor guidance for the repository

Acceptance Criteria

• pr-review-size.mdc or an equivalent persistent rule is added
• The three policy points above are documented clearly
• The work fits in one small, reviewable Pull Request

Proposed Plan

1. Check whether the repository already uses AGENTS.md or .cursor/rules
2. Add the PR size policy in the existing guidance structure
3. Verify the wording matches the organization-wide policy

Notes

• The same policy has already been added to other Jabelic-Works repositories