[UI] Name one way out of the stack preview

[KCaverly]

The split footer advertised both "d close" and "esc list" for the same
action, so it claimed two exits where there is one thing to do. esc is the
one worth naming: it is the step-out key everywhere else in the app, and it
is what the next change to this screen will build on.

d still toggles the preview shut — it is the key that opened it, and a
toggle that only opens would be its own surprise. It is simply no longer
advertised as a separate exit.

The label changed with it. "list" was already taken: in the diff-pane footer
tab is hinted as "list" and genuinely moves focus to the list pane, so an esc
sitting beside it under the same word read as a third way to move focus
rather than as the key that dismantles the split. "close preview" says what
it does.

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

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Stack (bottom → top):

1. [UI] Name one way out of the stack preview #43 ← this PR
2. [UI] Give d the preview and esc the way out #44
3. [UI] Drop the branch-diff jump from the stack screen #45
4. [UI] Stop the stack preview overrunning a narrow terminal #46
5. [Git] Pin the diff flags the body parser depends on #47
6. [UI] Teach both diff viewers the same file-jump key #48
7. [Diff] Thread the [diff] config through the viewer and zoom with z #49
8. [Git] Carry diff args and excludes through every diff call #50
9. [Diff] Add the external diff pager package #51
10. [Config] Accept a [diff] section with an optional pager #52
11. [Docs] Document the diff viewer settings #53