Add Rubber Duck debugging coverage to Module 2 workshop and slides

[Copilot]

This PR adds workshop coverage for the Copilot CLI Rubber Duck workflow so learners can use interactive chat to reason through problems step by step. It integrates the feature where operating modes are taught and keeps slide content aligned.

• Problem framing

  • Adds explicit guidance for using Copilot CLI as a “rubber duck” to clarify assumptions, surface unknowns, and drive diagnostic questioning during debugging.

• Workshop module updates (docs/workshop/02-modes.md)

  • Adds a new Rubber Duck Debugging in Interactive Mode subsection under Interactive Mode concepts.
  • Adds a concrete /ask side-question pattern for focused follow-ups.
  • Expands Exercise 4 (Interactive Mode Basics) with a rubber-duck prompt step.
  • Updates learning objectives and expected outcome language to include reasoning/clarification workflows.

• Slide sync updates (docs/slides/02-modes.md)

  • Adds a matching Rubber Duck in Interactive Mode slide with practical prompts.
  • Keeps module/slide narrative consistent around when to use interactive mode vs other modes.

• Example prompt pattern

  Rubber duck with me: I expected this script to parse JSON, but it fails on some inputs. Ask one diagnostic question at a time.
  /ask What JSON edge cases should I test first?