[UI] Show uncommitted work as a row in the stack

[KCaverly]

The stack screen could only show you commits, so the work in progress — the
thing most likely to be moving while you are looking at it — was the one
part of the worktree the preview pane could not reach. Reading it meant
leaving for the deck's diff viewer and its separate u scope toggle.

Uncommitted work now appears as a synthetic row trailing the commits, and
behaves like any other: it takes the cursor, drives the preview pane, and
windows with the rest. Commits are bottom-first, so it sits last, which is
where it belongs — above the tip.

The cursor now indexes a row list rather than status.Commits directly, which
is what makes that possible. clampStackCursor counts rows for the same
reason: clamping to the commit count would knock the cursor off the last row
on every passive refresh.

Contents are staged and unstaged together, matching what the deck already
counts as uncommitted, plus untracked file contents — git diff HEAD never
reports those, so a brand-new file would otherwise show as a name with
nothing in it. Untracked rendering is capped at 20 files, well clear of a
realistic worktree, and the surplus is named in the section rule rather than
silently dropped.

Caching splits by row kind. A commit's patch is immutable, so it is fetched
once and reused. Uncommitted work is the opposite — it changes under the
screen, which is the whole reason to watch it — so landing on the row always
revalidates, keeping the previous patch on screen while the new one is in
flight. Only the first landing shows "loading…", and the scroll position
carries across, or the row would be unreadable while an agent writes to the
worktree.

The row is hidden when the tree is clean rather than sitting there dead, and
o degrades to a flash: uncommitted work has no PR and cannot have one.

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

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Stack (bottom → top):

1. [Git] Add DiffUntracked to render untracked file contents #41
2. [UI] Show uncommitted work as a row in the stack #42 ← this PR
3. [UI] Name one way out of the stack preview #43
4. [UI] Give d the preview and esc the way out #44
5. [UI] Drop the branch-diff jump from the stack screen #45
6. [UI] Stop the stack preview overrunning a narrow terminal #46
7. [Git] Pin the diff flags the body parser depends on #47
8. [UI] Teach both diff viewers the same file-jump key #48
9. [Diff] Thread the [diff] config through the viewer and zoom with z #49
10. [Git] Carry diff args and excludes through every diff call #50
11. [Diff] Add the external diff pager package #51
12. [Config] Accept a [diff] section with an optional pager #52
13. [Docs] Document the diff viewer settings #53