#10779: Mandatory dupe check for PRs + Refine commit and PR guidelines in AGE…

…NTS.md Added mandatory dupe checks for PR and Issues. Consolidate commit and PR guidelines for clarity and consistency. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> - Updates `AGENTS.md` to tighten/clarify commit + PR guidance (commit message style, changelog ordering, review vs landing flow). - Adds an explicit, mandatory duplicate-search step before opening PRs/issues using `gh pr list -S` / `gh issue list -S`. - Refines PR landing instructions (temp branch, squash default, full gate before committing) and contributor crediting steps. - Pure documentation/guidelines change; no runtime code paths impacted. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge; it only adjusts contributor/agent guidelines in AGENTS.md. - Changes are limited to documentation and do not affect build/runtime behavior; no broken links or contradictory instructions were found within the updated section. - No files require special attention <!-- greptile_other_comments_section --> <!-- /greptile_comment -->