docs: add agent rule for changelog fragment content

[ogenstad]

Why

Changelog fragments regularly get written as if the reader had the diff in front of them: module
paths, class names, and the shape of the fix. Towncrier publishes those files verbatim in the
release notes, where the audience is people running Infrahub, not the people who wrote the patch.

The creating-changelog-entries skill covers the mechanics (naming, change types,
towncrier create), but nothing is attached to the files themselves at edit time.

What changed

• New rule .agents/rules/changelog-entries.md, scoped to changelog/*.md, so it loads whenever
  a fragment is being written or edited.
• The rule covers content only and defers mechanics to the existing skill and
  docs/docs/development/changelog.mdx rather than duplicating them:
  • Write for the user, not the reviewer - frame the change against the UI, API, CLI,
    branches, schema, or upgrade; for a fix, what looked broken from the outside.
  • Leave the implementation out - with avoid/prefer examples.
  • Say why it matters when it isn't obvious - one extra sentence for impact, explicitly not
    license to pad a one-line fix.
  • Style - past tense, lead with the change, one sentence by default, and no issue
    references in the body since the filename carries them and Towncrier renders the link.

.claude/rules is a symlink to .agents/rules, so no adapter mirroring was needed.

How to review

Read the rule against the fragments currently in changelog/ - they are the examples the
guidance was calibrated against.

Impact & rollout

Documentation for AI coding agents only. No product code, no behavior change.

Checklist

• Tests added/updated
• Changelog entry added (uv run towncrier create ...)
• External docs updated (if user-facing or ops-facing change)
• Internal .md docs updated (internal knowledge and AI code tools knowledge)
• I have reviewed AI generated content

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

<sub>Shadow auto-approve: would auto-approve. Adds a documentation-only rule file guiding AI agents on writing user-facing changelog fragments. No product behavior, contracts, or operations change; it is bounded documentation tooling.

Re-trigger cubic</sub>