Remove 22 !important declarations from mobileChatShell.css where the
.phone-layout class already wins by specificity. Kept !important only
where it fights SplitView inline styles, Part.layoutContents inline
sizing, or an equal-specificity desktop rule. Added a top-of-file
policy comment documenting the three legitimate reasons to use
!important here so future additions don't accrete out of habit.
Verified against a 393x852 iPhone emulation with the mock agent host:
welcome screen, sidebar overlay, session opening, and chat input
layout are unchanged.
Also updated the vscode-dev-workbench skill to call out that npm run
dev must run in the vscode-dev folder (not vscode) and that the mock
agent host is a separate process that must be started in addition to
the dev server.
