fix: nest model under engine: in PR reviewer YAML example

[Copilot]

model: was used as an invalid top-level frontmatter field in the annotated YAML example in workshop/11c-build-pr-reviewer.md. Per the agentic workflow syntax reference, model must be a sub-field of engine:.

Changes

• workshop/11c-build-pr-reviewer.md: Replace top-level model: gpt-4o with a proper engine: block; add a clarifying note that model is a sub-field of engine:, not a top-level key.

# Before
model: gpt-4o # LLM used for the review

# After
engine:
  id: copilot
  model: gpt-4o   # LLM used for the review

[pelikhan]

@copilot suggest other model, newer

[Copilot]

Updated to gpt-4.1 in commit 449e818's follow-up — it's the current successor to gpt-4o for the Copilot engine.

[pelikhan]

@copilot use a gpt-5.4-mini

[Copilot]

Updated to gpt-5.4-mini in the latest commit.