Point contributors at .nextchanges in PR template and pr-checklist

Update the contributor-facing guidance to add a .nextchanges/<section>/<name>.md fragment instead of editing NEXT_CHANGELOG.md: each PR adds its own file so entries never conflict, the filename is arbitrary, the leading bullet and the PR link are optional, and it can be created from the GitHub UI. These are the only two docs that described the old workflow.

Co-authored-by: Isaac

Author: janniklasrose

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

@@ -55,7 +55,7 @@ If an agent (you) authored or substantially helped author the PR, disclose it on
 
 ## 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.
+Add a changelog fragment under `.nextchanges/` when your change is user-visible. Each PR adds its own file, so entries never conflict between PRs. CI collates the fragments and generates the real `CHANGELOG.md` at release time, so never hand-edit `CHANGELOG.md` or `NEXT_CHANGELOG.md` directly.
 
 **When to add an entry:**
 - New or changed CLI command, flag, or subcommand behavior
@@ -69,7 +69,7 @@ Add a `NEXT_CHANGELOG.md` entry when your change is user-visible. CI generates t
 - Auto-generated output changes without a corresponding user-facing change
 
 **How to add:**
-- Pick the right section (`CLI`, `Bundles`, `Dependency updates`) under the current `## Release vX.Y.Z` header.
-- One or two sentences, user-facing language, no Jira links.
-- Reference the PR number once it's open: after `gh pr create`, edit the entry to append ` (#NNNN)` or similar matching nearby entries.
-- Match the voice and tense of the existing entries in the file.
+- Create `.nextchanges/<section>/<name>.md`, picking the section folder that fits: `cli`, `bundles`, `dependency-updates`, `notable-changes`, or `api-changes`. `<name>` is arbitrary (a feature name or your PR number) — just keep it unique. You can create it straight from the GitHub UI.
+- Write one or two sentences in user-facing language, no Jira links. The leading `* ` is optional. Match the voice and tense of existing changelog entries.
+- A PR link is optional: write `(#NNNN)` in the text and it's expanded to a full link automatically.
+- See `.nextchanges/README.md` for details.

.github/PULL_REQUEST_TEMPLATE.md

@@ -9,4 +9,5 @@ For example, were there any decisions behind the change that are not reflected i
 <!-- How have you tested the changes? -->
 
 <!-- If your PR needs to be included in the release notes for next release,
-add a separate entry in NEXT_CHANGELOG.md as part of your PR. -->
+add a changelog fragment: create .nextchanges/<section>/<name>.md with a
+one-line description (e.g. .nextchanges/cli/quickstart.md). See .nextchanges/README.md. -->