Add breeze issues unassign command

[potiuk]

Add a new breeze issues command group with an unassign subcommand that finds
open issues with assignees who are not repository collaborators and removes them.
This helps maintainers clean up stale issue assignments.

Features:

• Fetches collaborator list once for O(1) lookup per assignee
• Batched processing with configurable --batch-size
• --dry-run mode to preview without modifying (still authenticates to GitHub)
• Interactive confirmation per batch (yes/no/quit)
• Token resolution: --github-token flag → GITHUB_TOKEN env → gh auth token CLI

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Opus 4.6

Generated-by: Claude Opus 4.6 following the guidelines

[github-actions]

Backport failed to create: v3-1-test. View the failure log Run details

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
❌	v3-1-test

You can attempt to backport this manually by running:

cherry_picker 92954d8 v3-1-test

This should apply the commit to the v3-1-test branch and leave the commit in conflict state marking
the files that need manual conflict resolution.

After you have resolved the conflicts, you can continue the backport process by running:

cherry_picker --continue

If you don't have cherry-picker installed, see the installation guide.