[UI] Give d the preview and esc the way out

[KCaverly]

esc was doing two jobs on the stack screen. With the preview open it closed
the preview; with it shut it closed the screen. So leaving cost one keypress
or two depending on a mode you had to remember you were in, and d — the key
that opened the preview — shared the first job with it.

Now each key means one thing. d toggles the preview, open and shut, from
either pane. esc leaves the stack screen from anywhere and hands back
whatever was underneath.

The return itself already worked: the stack screen is an overlay and never
owned m.screen, so dropping it has always revealed the screen it was opened
over. What was wrong was the footer calling that "deck", which is only true
when the deck is what opened it — open the screen from the palette while on
the agent screen and esc has always gone back there. The hint now says
"return", which is true from every entry point, and the text-window footer
matches.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

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Stack (bottom → top):

1. [UI] Give d the preview and esc the way out #44 ← this PR
2. [UI] Drop the branch-diff jump from the stack screen #45
3. [UI] Stop the stack preview overrunning a narrow terminal #46
4. [Git] Pin the diff flags the body parser depends on #47
5. [UI] Teach both diff viewers the same file-jump key #48
6. [Diff] Thread the [diff] config through the viewer and zoom with z #49
7. [Git] Carry diff args and excludes through every diff call #50
8. [Diff] Add the external diff pager package #51
9. [Config] Accept a [diff] section with an optional pager #52
10. [Docs] Document the diff viewer settings #53