feat(subdomain): write endpoints — add / update / delete / move

[chmmou]

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

KAS actions in scope

• add_subdomain
• update_subdomain
• delete_subdomain
• move_subdomain

Notes

• Fixtures already in testdata/subdomain/ (PR test: restructure KAS API fixtures into per-package folders #22).
• delete_subdomain and move_subdomain are destructive — depend on the confirmation infrastructure in feat(cli): destructive-write confirmation infrastructure (--yes / interactive prompt) #109.
• Read counterpart get_subdomains is already wired.

Acceptance

• SOAP request encoder + per-action mapping in internal/subdomain/
• *Client write methods + per-package Caller interface tests
• CLI subcommands wired in internal/cli/subdomain.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