Update assign-reviewers workflow for fork PR support (#51876)

## Summary

Deploy updated `assign-reviewers.yml` from
[codeowner-coordinator#83](https://github.com/zed-industries/codeowner-coordinator/pull/83):

- Switch trigger from `pull_request` to `pull_request_target` to support
fork PRs and fix `author_association` misclassification bug (org members
reported as `COLLABORATOR`)
- Remove `author_association` filter and fork gate — reviewer
assignments are inherently scoped to org team members by the GitHub
Teams API
- Remove `--min-association member` from script invocation
- Add `SECURITY INVARIANTS` comment block documenting
`pull_request_target` safety requirements
- Add concurrency guard to prevent duplicate runs per PR
- Add `--require-hashes` + SHA256 pin for pyyaml install

## Test plan

- [ ] Verify a fork PR triggers the workflow and receives team
assignment
- [ ] Verify a draft→ready PR triggers correctly
- [ ] Verify org member PRs continue to work

Release Notes:

- N/A

.github/pull_request_template.md

@@ -1,10 +1,28 @@
-Closes #ISSUE
+## Context
 
-Before you mark this PR as ready for review, make sure that you have:
-- [ ] Added a solid test coverage and/or screenshots from doing manual testing
-- [ ] Done a self-review taking into account security and performance aspects
-- [ ] Aligned any UI changes with the [UI checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
+<!-- What does this PR do, and why? How is it expected to impact users?
+     Not just what changed, but what motivated it and why this approach.
+
+     Link to Linear issue (e.g., ENG-123) or GitHub issue (e.g., Closes #456)
+     if one exists — helps with traceability. -->
+
+## How to Review
+
+<!-- Help reviewers focus their attention:
+     - For small PRs: note what to focus on (e.g., "error handling in foo.rs")
+     - For large PRs (>400 LOC): provide a guided tour — numbered list of
+       files/commits to read in order. (The `large-pr` label is applied automatically.)
+     - See the review process guidelines for comment conventions -->
+
+## Self-Review Checklist
+
+<!-- Check before requesting review: -->
+- [ ] I've reviewed my own diff for quality, security, and reliability
+- [ ] Unsafe blocks (if any) have justifying comments
+- [ ] The content is consistent with the [UI/UX checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
+- [ ] Tests cover the new/changed behavior
+- [ ] Performance impact has been considered and is acceptable
 
 Release Notes:
 
-- N/A *or* Added/Fixed/Improved ...
+- N/A or Added/Fixed/Improved ...