[Diff] Add the external diff pager package

[KCaverly]

diffpager pipes each file's patch through an external formatter (delta,
diff-so-fancy, bat) while ct keeps ownership of scrolling and keybindings.
A zero-value Pager is inert, so the default stays ct's built-in styling.

The body is split on diff --git boundaries and each file rendered as its
own Chunk, which is what preserves the viewer's file-jump index. Headers
are matched against ANSI-stripped text so a user running --color=always
does not collapse the whole patch into one chunk and lose that index.

Rendering is all-or-nothing: the caller's fallback is to re-render with
ct's built-in styling, and a half-formatted patch would be worse than
either. Chunks render concurrently under a worker cap, since a 40-file
diff spawning processes serially would cost 40x a single process's
latency on the fetch path. A 30s timeout bounds the whole call so a
formatter that hangs cannot strand the fetch goroutine, and the first
failure cancels the rest rather than waiting out the budget.

cmd.Dir is set to the repo so a formatter reading .gitattributes or git
config sees the right repo, and COLUMNS is set because several formatters
size their output from it when stdout is not a terminal — which, piped
into a Builder, it never is.

Co-Authored-By: Claude Opus 5 noreply@anthropic.com

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Stack (bottom → top):

1. [Diff] Add the external diff pager package #51 ← this PR
2. [Diff] Thread the [diff] config through the viewer and zoom with z #49
3. [Docs] Document the diff viewer settings #53