feat(softwareinstall): add_softwareinstall

[chmmou]

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

KAS actions in scope

• add_softwareinstall (no update/delete counterparts in the KAS API; uninstall is a manual filesystem step)

Notes

• Fixtures already in testdata/softwareinstall/ (PR test: restructure KAS API fixtures into per-package folders #22).
• The action is potentially destructive (it writes/overwrites files in the document root) — depends on the confirmation infrastructure in feat(cli): destructive-write confirmation infrastructure (--yes / interactive prompt) #109. Warrants a loud prompt explaining what will be installed and where.
• Read counterpart get_softwareinstall_status (or equivalent) is already wired.

Acceptance

• SOAP request encoder + per-action mapping in internal/softwareinstall/
• *Client write method + per-package Caller interface tests
• CLI subcommand wired in internal/cli/softwareinstall.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