feat: add PR reviewer patterns as bridge between workshop Part 1 and Part 2

[Copilot]

Adds a new step 13 and three side quests covering common PR automation patterns. These sit between Part 1 (scheduled workflows) and Part 2 (advanced topics), introducing event-driven pull_request triggers before learners hit conditional logic and data sources.

New content

• 13-pr-reviewer-workflow.md — core step; builds a pull_request-triggered workflow that posts a review comment. Teaches the shift from schedule to event-driven triggers, pull-requests: write permission, and safe-outputs: create-issue-comment: limit: 1 to cap comment spam.

• side-quest-13-01-pr-labeler-pattern.md — maps file-path patterns to labels via add-labels-to-pull-request; includes a hands-on exercise extending the rule set.

• side-quest-13-02-pr-summary-pattern.md — posts a structured, changelog-ready summary on opened only (not synchronize); learners adapt the output template to a real use case.

• side-quest-13-03-pr-checklist-pattern.md — evaluates PRs against a quality checklist (description, scope, tests, docs, size) and posts a ✅/⚠️ table; refreshes on every push.

Wiring

• Step 12's "Next" link updated to point to step 13.
• README.md curriculum table and side quests section updated.
• Simulator step alias map and ordering updated for 13-pr-reviewer-workflow.