fix: fold demo learnings (strict-mode F&O error text, delete_groups measured, show-in-menu column) (5.8.6)

[Justinvolved]

Three demo-skill fixes from learning triage.

• dw-demo-fo: Get-FoErrorText read ErrorDetails.Message with no null check. Under Set-StrictMode -Version Latest a 404 with no body threw inside the caller's catch block, so Import-GoldenPackage.ps1 stopped before it created a missing import project. It now handles every error shape and falls back to the body text or the exception message. New hermetic suite tests/Fo.Api.Tests.ps1 (9 cases; 4 failed on the old code, all pass now).
• dw-demo-pim canonical-setup-order.md: delete_groups measured on DW 10.28.12 with MCP 0.6.0. It deletes no products (one group or two), and it leaves the group's product relations and shop relation as orphans that a same-id group picks up. Step 10 says to read stockStatus back, because the create response echoes neverOutOfStock wrongly.
• dw-demo-swift cheat-sheet.md: save_pages {showInMenu:false} writes PageActive, like set_page_menu. In the 0.6.0 and 0.6.1 add-in source, PageCreateModel and PageMenuModel both map ShowInMenu to Page.Active, and no tool writes PageShowInLegend.

Validation: python3 scripts/validate-skills.py gives 0 errors. node scripts/build-manifest.mjs was run. Invoke-Pester tests/Fo.Api.Tests.ps1 gives 9 passed.