Refine AGENTS.md changelog guidance: silence Codex noise and reserve .changed for breaking changes

[Kyle-Neale]

What does this PR do?

Two related refinements to the changelog guidance in the top-level AGENTS.md:

1. Adds a Review Guidelines section instructing the Codex code-review bot (chatgpt-codex-connector[bot]) not to raise a finding for a missing changelog entry.
2. Tightens the changed entry-type definition to backward-incompatible changes only, and adds a Choosing .changed guardrail that tells agents to pause and confirm a change is genuinely breaking before using .changed.

The changelog requirement for authored code is unchanged.

Motivation

Both changes address feedback from Eric Weaver in #agent-integrations Slack. AGENTS.md is the single source both Codex and Claude read for changelog behavior, so tightening the guidance here reshapes both agents at once.

Codex missing-changelog noise. Codex derives its review rules from AGENTS.md, so the existing "changelog entries are required" rule makes it post a finding asking for a changelog on essentially every non-draft PR (e.g. this P2 on #24243). That finding is both unavoidable and redundant:

• Changelog files are named <INTEGRATION>/changelog.d/<PR_NUMBER>.<TYPE>, so they can only be created after the PR number is assigned. Our own AGENTS.md tells contributors to open the PR first and add the entry afterward, so the entry is legitimately absent when the PR is first reviewed.
• The requirement is already enforced by the check_changelog job in .github/workflows/pr-quick-check.yml.

Over-use of .changed. The changed type was defined as "Breaking changes or significant modifications", and that loose "or significant modifications" phrasing led agents to tag non-breaking work as .changed (which bumps the major version). The definition is now backward-incompatible-only, with an explicit guardrail to stop and reconsider before choosing it.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

🤖 Generated with Claude Code

[dd-octo-sts]

Validation Report

All 21 validations passed.

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Validation	Description	Status
agent-reqs	Verify check versions match the Agent requirements file	✅
ci	Validate CI configuration and code coverage settings	✅
codeowners	Validate every integration has a CODEOWNERS entry	✅
config	Validate default configuration files against spec.yaml	✅
dep	Verify dependency pins are consistent and Agent-compatible	✅
http	Validate integrations use the HTTP wrapper correctly	✅
imports	Validate check imports do not use deprecated modules	✅
integration-style	Validate check code style conventions	✅
jmx-metrics	Validate JMX metrics definition files and config	✅
labeler	Validate PR labeler config matches integration directories	✅
legacy-signature	Validate no integration uses the legacy Agent check signature	✅
license-headers	Validate Python files have proper license headers	✅
licenses	Validate third-party license attribution list	✅
metadata	Validate metadata.csv metric definitions	✅
models	Validate configuration data models match spec.yaml	✅
openmetrics	Validate OpenMetrics integrations disable the metric limit	✅
package	Validate Python package metadata and naming	✅
qa-label	Validate the pull request declares whether it needs QA for the next Agent release	✅
readmes	Validate README files have required sections	✅
saved-views	Validate saved view JSON file structure and fields	✅
version	Validate version consistency between package and changelog	✅

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