Add merge-conflict marker check to PR template and contributing guide

[mdheller]

Motivation

• Ensure PR authors and contributors run a simple check to catch leftover Git merge conflict markers before committing or opening a PR.

Description

• Added a checklist item and a repository check command for merge-conflict markers to .github/PULL_REQUEST_TEMPLATE.md and CONTRIBUTING.md by inserting - [ ] No Git merge conflict markers remain in touched files and the git grep -nE '^(<{7}|={7}|>{7})' -- . validation step into the validation commands section.

Testing

• Ran the new merge-conflict check git grep -nE '^(<{7}|={7}|>{7})' -- . against the repository and it returned no matches, indicating no leftover conflict markers were present.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Invert merge-marker validation command

git grep -nE '^(<{7}|={7}|>{7})' -- . returns exit code 1 when no matches are found and 0 when conflict markers are present, so as written this “validation” step fails on clean trees and can pass when markers exist. This is especially problematic if contributors run the listed commands in a set -e shell or CI-style script, because the check’s success condition is backwards.

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