[Config] Accept a [diff] section with an optional pager

[KCaverly]

Adds the Diff config: args and exclude, threaded into repo.DiffOptions,
plus a [diff.pager] subtable naming an external formatter each file's
patch is piped through. Omitting the subtable keeps ct's built-in
styling, which stays the default.

A literal {width} in the pager args is replaced with the viewer's current
column count, so a formatter can be sized to the pane.

This is the commit that restores the build: the TUI wiring in ea96527
imports internal/diffpager and reads cfg.Diff.Pager, neither of which
existed in the tree until now.

Covers the config parsing and the chunk-rendering path in the viewer.

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

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Stack (bottom → top):

1. [Diff] Thread the [diff] config through the viewer and zoom with z #49
2. [Git] Carry diff args and excludes through every diff call #50
3. [Diff] Add the external diff pager package #51
4. [Config] Accept a [diff] section with an optional pager #52 ← this PR
5. [Docs] Document the diff viewer settings #53

[KCaverly]

Folded into #49. This commit's tests call m.diffOptions() and m.diffPager() from the viewer, while the viewer reads cfg.Diff.Pager defined here — a genuine cycle that no ordering resolves. Squashed together to get the stack building; no content dropped.