feat(mail): mailaccount write endpoints — add / update / delete

[chmmou]

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

KAS actions in scope

• add_mailaccount
• update_mailaccount
• delete_mailaccount

Notes

• Fixtures already in testdata/mailaccount/ (PR test: restructure KAS API fixtures into per-package folders #22).
• delete_mailaccount is destructive — depends on the confirmation infrastructure in feat(cli): destructive-write confirmation infrastructure (--yes / interactive prompt) #109.
• Read counterpart get_mailaccounts is already wired.

Acceptance

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