Add welcome bot for first-time contributors

[zeevdr]

Description

Automatically thank first-time contributors when they open their first issue or PR. Makes the project feel welcoming without manual effort.

Acceptance criteria

• Add GitHub Action (e.g., first-interaction action) to decree repo
• Configure welcome message for first issue
• Configure welcome message for first PR
• Replicate to SDK and UI repos (or use org-level workflow after migration)

Notes

• Keep messages short and genuine — no corporate boilerplate
• Point first-time issue authors to CONTRIBUTING.md and Discussions
• Point first-time PR authors to the testing/review expectations