feat(tui): add last turn source to diff viewer

[nexxeln]

What

Adds a Last turn source to the TUI diff viewer, backed by the session.diff route from #47821. It shows the files changed since the last prompt (everything between idle markers), alongside the existing All / Committed / Uncommitted VCS sources.

How

• packages/tui/src/config/index.tsx: diffs.source accepts turn (new DiffSource union over Vcs.Mode).
• packages/tui/src/feature-plugins/system/diff-viewer.tsx: the source picker offers Last turn only when the viewer was opened from a session. Selecting it calls client.session.diff({ sessionID, context: 12 }) and feeds the result through the same normalizeDiffs path, so empty results and errors reuse the existing states. Turn skips base resolution (needsBase). A remembered or configured turn source outside a session falls back to branch.
• Docs: services/www/src/docs/content/cli/config.mdx lists the new value.

Tests

packages/tui: test/cli/tui/diff-viewer.test.tsx gains three cases (session.diff request shape, empty/error states, option absent and fallback outside a session); test/config-v2.test.tsx accepts the new value. 113 pass. bun typecheck and prettier clean.

Not live-tested against a model; verified via tests only.