feat(mail): mailstandardfilter write endpoints — add / delete

[chmmou]

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

KAS actions in scope

• add_mailstandardfilter
• delete_mailstandardfilter

Notes

• Fixtures already in testdata/mailstandardfilter/ (PR test: restructure KAS API fixtures into per-package folders #22).
• delete_mailstandardfilter is destructive — depends on the confirmation infrastructure in feat(cli): destructive-write confirmation infrastructure (--yes / interactive prompt) #109.
• The KAS API has no update_mailstandardfilter — modification is an add+delete pair (callers handle).
• Read counterpart get_mailstandardfilter is already wired.

Acceptance

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