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>

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