feat(ci): add self-assign issue workflow

[0xMudit]

Summary

Fixes #3773

Adds a GitHub Actions workflow that lets contributors self-assign open issues by commenting \ ake\ and unassign with \untake.

• Triggers on \issue_comment\ events (created)
• Only runs on open, unassigned issues
• Exact keyword match: \ ake\ to assign, \untake\ to unassign
• Posts a confirmation comment after each action
• Uses \�ctions/github-script@v7\ (no external dependencies)

Verification

• Workflow YAML passes \�ctionlint\ syntax validation
• License header matches ASF convention used in other workflows

AI use

• Generative tooling made a substantive contribution

Tool(s) and scope: opencode/big-pickle — designed and wrote the workflow file

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — new CI workflow that responds to issue comments