pr-checklist: add PR description guidance for agents (#5105)

## Changes

Add a `## PR description` section to
`.agent/skills/pr-checklist/SKILL.md` telling agents to:

- Follow `.github/PULL_REQUEST_TEMPLATE.md` exactly — same headings (`##
Changes`, `## Why`, `## Tests`), same order, no invented sections like
`## Summary` / `## Test plan`, no leftover HTML comment placeholders.
- Read the template before passing `--body` to `gh pr create`.
- Disclose agent involvement on the last line of the body (e.g. `_This
PR was written by Claude Code._`), with the level of involvement
honestly reflected.

## Why

Agents drafting PRs in this repo have been freelancing the structure —
emitting `## Summary` / `## Test plan` headings (the generic Claude Code
defaults) instead of the repo's `## Changes` / `## Why` / `## Tests`
template, leaving HTML comment placeholders in, or omitting the
agent-authorship disclosure. Encoding this in the pre-PR checklist skill
means the agent picks up the rule the same way it picks up the lint/test
commands, instead of relying on reviewers to catch it after the fact.

## Tests

Docs-only change to a skill markdown file; no automated tests apply.

_This PR was written by Claude Code._

Author: janniklasrose

.agent/skills/pr-checklist/SKILL.md

@@ -42,6 +42,14 @@ After the commands above pass, scrub the diff before pushing. The quick version:
 - **TODOs without a ticket**: either add a ticket reference (e.g. `// TODO(DECO-1234): ...`) or remove the TODO. Un-tracked TODOs rot.
 - **Unintended files**: review `git status` and `git diff --stat` to confirm only the files you meant to change are staged.
 
+## PR description
+
+Follow `.github/PULL_REQUEST_TEMPLATE.md` exactly. Use its section headings (`## Changes`, `## Why`, `## Tests`) in the same order, and fill each one in. Do not invent new sections (`## Summary`, `## Test plan`, etc.), do not drop sections, and do not leave the HTML comment placeholders in the final body — replace them with real content. If a section genuinely does not apply (e.g. a docs-only change has no test steps), say so explicitly under that heading rather than removing it.
+
+When using `gh pr create`, read `.github/PULL_REQUEST_TEMPLATE.md` first and base `--body` on it.
+
+If an agent (you) authored or substantially helped author the PR, disclose it on the last line of the body, e.g. `_This PR was written by Claude Code._` or `_PR description drafted with Claude Code._`. Be honest about the level of involvement — "written by" vs. "drafted with" vs. "reviewed by" — and keep it to a single italicized line so it doesn't crowd the template sections.
+
 ## Changelog entry
 
 Add a `NEXT_CHANGELOG.md` entry when your change is user-visible. CI generates the real `CHANGELOG.md` from `NEXT_CHANGELOG.md` at release time, so never hand-edit `CHANGELOG.md` directly.