docs(skills/summon): Clarify destination is the left-hand session picker

The "sidebar" wording was ambiguous — Desktop has multiple panes.
Make it explicit that summoned sessions appear in the left-hand
session picker on Desktop's Code tab.

Description leads with neutral "Transfer" verb and presents push and
pull as two equal framings, recommending push without forcing it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>