chore: add issue template for good first issue suggestions

[alexar76]

Why

We have bug/feature templates; external contributors need a lightweight way to propose small tasks.

Task

Add .github/ISSUE_TEMPLATE/good_first_issue.yml with fields:

• Suggested title
• Repo (aicom / metis / aimarket-mcp / other)
• Why it's small (≤1 day)
• Acceptance checklist (3 bullets)

Acceptance

• Template appears in GitHub New Issue UI
• Auto-applies labels good first issue + help wanted if GitHub allows, else document in body