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>

Author: adinauer

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

@@ -35,6 +35,8 @@ Derive the branch name from the changes being made. Use `feat/`, `fix/`, `ref/`,
 
 **For stacked PRs:** For the first PR in a new stack, first create and push the collection branch (see `.cursor/rules/pr.mdc` § "Creating the Collection Branch"), then branch the PR off it. For subsequent PRs, branch off the previous stack branch. Use the naming conventions from `.cursor/rules/pr.mdc` § "Branch Naming".
 
+**CRITICAL: Never merge, fast-forward, or push commits into the collection branch.** It stays at its initial position until the user merges stack PRs through GitHub. Updating it will auto-merge and destroy the entire PR stack.
+
 ## Step 2: Format Code and Regenerate API Files
 
 ```bash

.cursor/rules/pr.mdc

@@ -185,6 +185,8 @@ git push -u origin HEAD
 gh pr create --base main --draft --title "<type>(<scope>): <Topic>" --body "Collection PR for the <Topic> stack. Squash-merge this once all stack PRs are merged."
 ```
 
+**CRITICAL: Do NOT manually update the collection branch.** Never merge, fast-forward, or push stack branch commits into the collection branch. The collection branch stays at its initial position (the empty commit on `main`) until the user merges individual stack PRs into it one by one through GitHub. If you fast-forward the collection branch to include stack commits, GitHub will auto-merge and delete all stack PR branches, destroying the entire stack.
+
 ### Creating a New Stacked PR
 
 1. Start from the tip of the previous stack branch (or the collection branch for the first PR).
@@ -237,12 +239,12 @@ Once all stack PRs are merged into the collection branch, the collection PR is *
 
 ### Syncing the Stack
 
-When a base PR changes (e.g. after addressing review feedback on PR 1), merge the changes forward through the stack:
+When a base PR changes (e.g. after addressing review feedback on PR 1), merge the changes forward through the stack **between adjacent stack PR branches only**:
 
 ```bash
 # On the branch for PR 2
 git checkout feat/scope-attributes-logger
-git merge feat/scope-attributes
+git merge feat/scope-attributes-api
 git push
 
 # On the branch for PR 3
@@ -251,4 +253,6 @@ git merge feat/scope-attributes-logger
 git push
 ```
 
+**Never merge into the collection branch.** Syncing only happens between stack PR branches. The collection branch is untouched until the user merges PRs through GitHub.
+
 Prefer merge over rebase — it preserves commit history, doesn't invalidate existing review comments, and avoids the need for force-pushing. Only rebase if explicitly requested.