docs: rework workshop flow and refresh authoring tooling

[GeekTrainer]

Summary

Reworks the workshop flow across all three paths (Copilot CLI, VS Code, Cloud coding agent) and refreshes the authoring/contributor tooling and docs.

Changes

• Copilot CLI rework — always launch with allow-all (yolo), drop the Microsoft MCP server, put the GitHub MCP server in read-write with a callout, and reorder to issues-first then custom-instructions.
• VS Code & Cloud lessons — aligned flow and fixed cross-path bleedover/inconsistencies.
• Shared partials & landing — new callout-approve-run-workflows and exercise-review-accessibility-agent; updated overviews, landing, and prereqs.
• Image dir renamed images/ → _images/ so it's excluded from routing like other underscore dirs.
• Skills — added build-and-verify-docs and validate-site-playwright (+ skills index and .mcp.json); replaced the commit/PR skills with make-repo-contribution.
• Authoring docs — new accessibility and instruction-authoring guides; corrected false "CI runs the partial scripts" claims (CI runs only build + lychee; partial lint/sync are local guardrails).

Verification

• lint_partials.py: OK (17 partials / 24 consumers)
• sync_partial_metadata.py --check: in sync
• npm run build: 25 pages
• lychee offline: 0 errors