Flatten the bundled Matrix messaging skill layout

[TroyHernandez]

Summary

• Move the bundled Matrix messaging skill to inst/skills/matrix-messaging/ and remove the duplicated nested layout.
• Preserve the skill body exactly; no client runtime code or API changes.
• Document the direct-path migration and add seven installed-package assertions to prevent duplicate or missing skill packaging.

Validation

• Isolated source build and R CMD check --no-manual --no-vignettes: OK.
• Package skill discovery and exact instruction retrieval pass against saber 0.7.2.5 in an isolated library.
• Git identifies the skill as a 100% rename. Diff whitespace check is clean.
• No Matrix messages, service changes, normal installations or session restarts.