docs: require issue resolution evidence

[LauraGPT]

Summary

• document that a merged PR, green main branch, or available release is not sufficient evidence to close a user-reported issue
• require reporter confirmation or maintainer reproduction plus released-version verification and a reasonable feedback window
• ask maintainers to link the release and keep the issue in a waiting-for-feedback state instead of treating silence as confirmation

Validation

• git diff --check
• verified this PR changes only CONTRIBUTING.md
• verified the resolution section contains the reporter-confirmation, released-version, evidence, and feedback-window requirements

Signed commit with DCO.