Policy: Require PR↔Issue linkage and 'Closes #…' in PRs

[flyingrobots]

Adopt a repo-wide policy that every PR must be tied to a GitHub Issue, and PR descriptions must include a closing keyword to auto-close the issue on merge.\n\nScope\n- Update AGENTS.md with the policy and process.\n- Ensure future PRs use 'Closes #' in the body.\n- Encourage 1 PR = 1 thing; open an issue first if one does not exist.\n\nAcceptance\n- AGENTS.md updated with the new section.\n- Announce in next PR description; optional: add a PR checklist reminder.\n