Adds a 'New session' header button in Mission Control that opens a folder picker
and starts a task-less embedded ConPTY session in the chosen directory. ConPtyPaneViewModel
TaskId is now nullable (ad-hoc panes have no task and are never deduped) with a
CreateAdHoc factory; the view does the picking, the VM stays picker-agnostic.
Adds an 'Open ConPTY session' entry that fetches a task's launch spec and hosts
an embedded ConPTY terminal as a Mission Control pane, coexisting with the
streamed-log monitor panes (streaming stack untouched). Introduces IMissionControlPane
+ ConPtyPaneViewModel, a non-destructive Panes mirror (Monitors prefix + ConPtySessions
suffix) so unrelated monitor churn never tears down a live terminal, and a grid<->tabs
layout toggle. Launch failures surface via the footer error strip.
mika kuns