full-day workshop: Refresh Copilot terminology and model-specific guidance

Problem

The full-day content hard-codes Copilot details that change independently of the workshop:

• 5-context.md focuses on the @workspace participant and a free-trial signup flow.
• 6-code.md says repository custom instructions are available only to Chat.
• 6-code.md requires Edit mode and recommends the account-dependent Claude 3.7 Sonnet model.
• The devcontainer installs both GitHub.copilot and GitHub.copilot-chat, which should be checked against the current extension packaging.

These instructions can diverge from the stable VS Code experience based on extension version, account plan, organization policy, and model retirement.

Suggested change

Validate Exercises 3, 5, and 6 against the current stable Copilot experience. Prefer capability-based language and treat named models as optional examples.

Acceptance criteria

• Current mode, context, custom-instruction, and extension terminology is used consistently.
• Statements about where custom instructions apply are accurate.
• Model selection does not require a specific optional or retired model.
• Signup language reflects current Copilot access options.
• Screenshots and controls match a current stable VS Code installation.

Review Exercises 3, 5, and 6, especially 5-context.md, 6-code.md, and the devcontainer configuration. Compare their Copilot terminology, access guidance, model references, extensions, controls, and screenshots with a current stable VS Code installation. Done means the acceptance criteria are met without requiring a specific model or outdated access flow.