docs: skill version retention, skill edit fields and transferred skill files

[mrgoonie]

Summary

Documents the runtime changes in nextlevelbuilder/dewee for issues #1239, #1246, #1249, #1257, #1258 and #1260, in English and Vietnamese:

• Concepts → Skills: a new "Version retention" bullet covering:
  • which versions the gateway keeps, including pinned versions;
  • deleted skills' files being removed after 7 days;
  • interrupted uploads moving to the trash folder after 24 hours;
  • a deleted skill not being switchable back on;
  • the fields an API edit can change.
• Concepts → Multi-tenancy: a transferred custom skill takes every version of its files with it, and an existing version at the destination stops the transfer before anything is copied.

Test Plan

• The content is prose only. I checked each statement against the runtime code and the docs in the dewee PR.

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https://claude.ai/code/session_015f8sdk5oxpWydW2fHgdiVa

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