Extend InteractiveCommand with needs_object_pick / on_object_pick so a
plugin tool can prompt the user to pick an existing entity (handle + point)
rather than a free point — the structure-pick path Storm Sewer's SS_PIPE
needs to connect existing structures. The host adapter delegates to the
internal entity-pick flow; over --serve the pick is fed as a hex handle.
Stays API v2 (no released plugin implements InteractiveCommand yet).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
707c143212