[v2.3][Process] Complete templates + Mermaid class diagrams

[JasonEran]

Scope

Complete course/process documentation tasks under Epic #8.

Closes #16
Closes #17
Closes #18

What changed

• Added task issue template:
  • .github/ISSUE_TEMPLATE/task.md
• Extended bug/feature templates to include:
  • Acceptance Criteria
  • Validation Steps
• Expanded PR template with:
  • Linked Issue
  • explicit Validation section
  • Rollback Notes
  • security checklist item
• Added Mermaid class diagrams under docs/diagrams/:
  • docs/diagrams/Class-Diagram-v2.3.md
• Linked class-diagram doc in README docs index.

Validation

• Template coverage checks (bug/feature/task) and required sections verified.
• PR template fields verified (Linked Issue, Validation, Rollback Notes, security checklist).
• Mermaid class diagrams include key classes/relationships across Core/Agent/AI/Web.

Notes

• Epic [v2.3][Epic] Course + Project Management #8 remains open until [Process] Set up GitHub Project board (To do / In progress / Done) #19 (GitHub Project board setup) is confirmed/closed.