docs(controller): add PR motivation and second-eye review channel guide

[weicao]

Summary

• Add docs/controller/addon-controller-pr-author-motivation-and-review-channel-guide.md.
• Define the PR body opening rule: first explain the concrete problem, field example, and what would be misleading without the fix.
• Define the review-channel boundary: second-eye process notes stay in team discussion; GitHub PR comments keep maintainer-facing blockers, design decisions, and final evidence.
• Add the guide to docs/controller/README.md.

Why

Controller maintainers need PR bodies that explain why a change matters before they read code details. They also need GitHub PR comments to stay focused on actionable review material, not process noise from pair review or local verification handoffs.

Test plan

• addon-docs-writing self-check: problem-first opening, plain-language section, concise length, case appendix boundary.
• git diff --check origin/main...HEAD
• Related markdown links resolve in the changed guide.
• PR body, commit messages, and changed docs public-hygiene grep clean.