docs: refresh and localize App workshop

[GeekTrainer]

Summary

Refreshes the GitHub Copilot App workshop into a continuous two-PR Tailspin Toys workflow covering Plan and Autopilot modes, custom instructions, the quality-checks skill, Playwright MCP, a QA custom agent, Agent Merge, and canvas exploration. Synchronizes the complete App path across Spanish, Japanese, Korean, Brazilian Portuguese, and Simplified Chinese, and consolidates the repository's workshop-writing agents into a focused Tech Writer agent.

Type of change

• Workshop content (lesson Markdown, images)
• Site shell (website/ Astro + Starlight wrapper)
• Copilot configuration (.github/copilot-instructions.md, instructions, agents, skills)
• Repo housekeeping (CI, dependabot, README, license)
• Other:

Verification

• Type checks and a clean site build pass (230 pages, 0 errors).
• Affected pages and translations match their source and intended routes.
• Offline internal link checks pass (15,147 valid, 0 errors).
• Changed external links have been checked separately (23 targets, 0 failures).

Notes for reviewers

This updates the existing App workshop PR rather than opening a duplicate. The branch was merged with current origin/main (646a63f) without conflicts, then fully reverified.

[GeekTrainer]

Superseded by #186, which targets main directly after the refreshed branch was merged with current main and reverified. GitHub would not allow the base of this stacked PR to be changed.