Add contributor call agenda issue template

[jflo]

Summary

• Adds a new GitHub issue template for Besu contributor call agendas
• Includes standard housekeeping boilerplate (antitrust notice, recording disclosure, mute/raise-hand reminders)
• Pre-fills 3 agenda item placeholders for the creator to fill in
• Auto-assigns to jflo and applies the agenda label

Test plan

• Verify the template appears in the issue creation chooser at https://github.com/besu-eth/besu/issues/new/choose
• Create a test issue using the template and confirm formatting looks correct

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a GitHub Issue Template to standardize Besu contributor call agenda issues, including housekeeping boilerplate and pre-filled agenda placeholders.

Changes:

• Adds a new Markdown-based issue template for contributor call agendas
• Includes housekeeping items (antitrust, recording disclosure, meeting etiquette)
• Auto-applies the agenda label and assigns jflo