feat(ftpuser): write endpoints — add / update / delete

[chmmou]

Vertical-slice (per kasapi-cli-vertical-slice SKILL) for the FTP user write endpoints. Subset of #13.

KAS actions in scope

• add_ftpuser (KAS docs spell this add_ftpusers — plural; fixture uses singular. Resolve against the live API as the first step of this slice.)
• update_ftpuser
• delete_ftpuser

Notes

• Fixtures already in testdata/ftpuser/ (PR test: restructure KAS API fixtures into per-package folders #22).
• delete_ftpuser is destructive — depends on the confirmation infrastructure in feat(cli): destructive-write confirmation infrastructure (--yes / interactive prompt) #109.
• Plural-vs-singular discrepancy on add_ftpuser(s) should be confirmed by sending a real request and observing which action name the API accepts; document the result in CHANGELOG and adjust the fixture if the docs win.
• Read counterpart get_ftpusers is already wired.

Acceptance

• Plural-vs-singular question resolved + CHANGELOG note
• SOAP request encoder + per-action mapping in internal/ftpuser/
• *Client write methods + per-package Caller interface tests
• CLI subcommands wired in internal/cli/ftpuser.go, registered in cmd/kasapi-cli/main.go
• make docs regenerated and committed
• CHANGELOG ### Added entry
• PR references this issue with Closes #<n>

Parent: #13