docs: Improve stacked PR workflow for collection branch and merge method (#5178)

* docs: Add collection branch safety warnings to PR workflow

Add explicit warnings to pr.mdc and create-java-pr skill that the
collection branch must never be manually merged or fast-forwarded.
Updating it causes GitHub to auto-merge all stack PRs and delete
their branches, destroying the entire stack.

Co-Authored-By: Claude <noreply@anthropic.com>

* docs: Include collection branch PR in stack list updates

The stack list update instructions only mentioned "all PRs in the
stack" which was ambiguous — it could be read as excluding the
collection branch PR. Now both pr.mdc and the create-java-pr skill
explicitly call out that the collection branch PR must also be updated
with the stack list.

Co-Authored-By: Claude <noreply@anthropic.com>

* docs: Add merge commit reminder to stack PR descriptions

Stack PRs must be merged with merge commits, not squash. Add a
reminder to the PR description so reviewers/mergers pick the right
method. This only applies to stack PRs — not standalone PRs or the
collection branch PR.

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: adinauer

.claude/skills/create-java-pr/SKILL.md

@@ -113,14 +113,15 @@ Fill in each section based on the changes being PR'd. Check any checklist items
 - Pass `--base <previous-stack-branch>` so the PR targets the previous branch (first PR in a stack targets the collection branch).
 - Use the stacked PR title format: `<type>(<scope>): [<Topic> <N>] <Subject>` (see `.cursor/rules/pr.mdc` § "PR Title Naming").
 - Include the stack list at the top of the PR body, before the `## :scroll: Description` section (see `.cursor/rules/pr.mdc` § "Stack List in PR Description" for the format).
+- Add a merge method reminder at the very end of the PR body (see `.cursor/rules/pr.mdc` § "Stack List in PR Description" for the exact text). This only applies to stack PRs, not the collection branch PR.
 
 Then continue to Step 5.5 (stacked PRs only) or Step 6.
 
 ## Step 5.5: Update Stack List on All PRs (stacked PRs only)
 
 Skip this step for standalone PRs.
 
-After creating the PR, update the PR description on **every other PR in the stack** so all PRs have the same up-to-date stack list. Follow the format and commands in `.cursor/rules/pr.mdc` § "Stack List in PR Description".
+After creating the PR, update the PR description on **every other PR in the stack — including the collection branch PR** — so all PRs have the same up-to-date stack list. Follow the format and commands in `.cursor/rules/pr.mdc` § "Stack List in PR Description".
 
 ## Step 6: Update Changelog
 

.cursor/rules/pr.mdc

@@ -199,7 +199,7 @@ gh pr create --base main --draft --title "<type>(<scope>): <Topic>" --body "Coll
 
 ### Stack List in PR Description
 
-Every PR in the stack must have a stack list **at the top of its description** (before the `## :scroll: Description` section). When a new PR is added, update the description on **all** PRs in the stack.
+Every PR in the stack — **including the collection branch PR** — must have a stack list **at the top of its description** (before the `## :scroll: Description` section). When a new PR is added, update the description on **all** PRs in the stack and on the collection branch PR.
 
 Format:
 
@@ -215,6 +215,14 @@ Format:
 
 No status column — GitHub already shows that. The `---` separates the stack list from the rest of the PR description.
 
+**Merge method reminder:** On stack PRs (not the collection branch PR), add the following line at the very end of the PR description:
+
+```markdown
+> ⚠️ **Merge this PR using a merge commit** (not squash). Only the collection branch is squash-merged into main.
+```
+
+This does not apply to standalone PRs or the collection branch PR.
+
 To update the PR description, use `--body-file` to avoid shell quoting issues with special characters in the body:
 
 ```bash