Align contributor onboarding with the development guidance #125

What is this about

The contributor guide's review instructions miss feedback outside inline threads.

Context

The guide treats unresolved inline threads as the only actionable feedback. Its commit and PR-body rules also need the changes in cozystack/cozystack#4859 once that PR lands.

What would help

Keep the onboarding instructions aligned with the main guide. Update contributors/README.md after that PR lands:

• Separate commit-reference and issue-closing conventions, including qualified references to issues in other repositories.
• Require issue links in the PR body and explain the default-branch requirement for automatic linking and closure.
• Cover review bodies, conversation comments and direct commit comments as well as inline threads, including automated feedback. Keep raw Markdown so link destinations remain available.