Slide the main screen over a session with a right swipe

Switching conversation was a step back to the list and a step down into
another, which disposed the session being left and refetched its whole
transcript over the tunnel on the way back. A right swipe now pulls
MainScreen itself over the open session -- the screen Back would have
shown, moved over the session instead of replacing it -- and swiping it
back off returns to a live stream, an unsent draft and the scroll
position it had. Tapping the session already open is that same swipe
back; tapping another is a screen of its own; deleting the one
underneath closes the screen, since there is nothing left to return to.

One gesture drives both this and the subagent panel (SidePanels.kt, now
the home of the drag and animation SubagentPanel had): two draggables
over the same content cannot share a horizontal drag, so the position is
a single signed reveal, negative left and positive right, which also
makes it impossible to have both open. The panels exist only inside a
session, so nothing on the main screen swipes anywhere.

Full width and no tonal step for this one, because a screen standing in
for another must be the same colour as it; the subagent panel keeps its
88% and its sliver. The list keeps its rows while it asks again -- the
panel refetches on every open, and blanking it each time handed the
reader an empty screen about something never in doubt -- with a bar over
the top while an answer is outstanding.

Where the panel has got to is read from draw lambdas only: it changes
every frame of a drag, and a body that reads it recomposes the session
beneath once per frame. Composition sees booleans that change twice per
gesture, the same correction the keyboard inset needed.

Verified on the emulator against the sandbox with ui-trace: the panel
opens and closes on the two swipes, tapping another session replaces the
screen, deleting the open one leaves for the list, the subagent panel is
unchanged, and neither swipe does anything on the main screen. ktfmt,
compile, lint and the unit tests are clean.

AGENTS.md

@@ -136,7 +136,10 @@ Module-by-module intent is in PLAN.md's "Backend layout".
   state and thread id, and reads subscription limits through the same CLI
   protocol.
 - `app/` — the Compose app, package `com.example.aiapp`, label "AI Sessions".
-  `AppRoot.kt` is the navigation `when`; `MainScreen.kt` the root's three tabs
+  `AppRoot.kt` is the navigation `when`; `SidePanels.kt` the one drag that
+  slides the whole main screen over a session from the left (`MainPanel.kt`)
+  and its subagents over it from the right (`SubagentPanel.kt`), both keeping
+  the session composed underneath; `MainScreen.kt` the root's three tabs
   (sessions, import, machines); `MachineModels.kt` the models on one machine
   and the downloads putting them there, drawn inside `ProviderScreen.kt` for a
   provider that serves files off that machine's disk; `Api.kt`/`EventStream.kt`