Add stale branch cleanup workflow and lifecycle policy

[HariniMalothu17]

Description

Introduces a GitHub Action to automatically clean up stale branches and a documentation page outlining the branch lifecycle policy.

Changes

• cleanup-stale-branches.yml

• Scheduled to run every Monday at 08:00 UTC — automatically deletes stale branches

• Can also be triggered manually with a dry-run option for previewing

• Two-tier staleness thresholds:

copilot/* branches: 90 days
All other branches: 180 days
Protected branches (never deleted):
main, master, develop
release/, hotfix/, archive/*
-stable (e.g., 0.72-stable, 0.80-stable)
preview- (e.g., preview-0.80-test)

• Skips branches with open PRs (including drafts)

• If you need to keep a branch from being deleted, Rename it under archive/

• branch-lifecycle-policy.md

Documents the cleanup policy, thresholds, and protected patterns
Explains how to preserve a branch from cleanup
FAQ for common questions

Motivation
The repository has accumulated many stale branches over time. This workflow helps maintain a clean branch list by automatically removing inactive branches while protecting important ones (stable releases, previews, branches with open PRs).

Testing
Ran the workflow manually in dry-run mode — completed successfully in 20s
Verified protected branch patterns against existing repo branches

Type of Change

• Automation (AI changes or Github Actions to reduce effort of manual tasks)

Resolves #15034

Microsoft Reviewers: Open in CodeFlow