Keep historical chat diffs inspectable

[jsgrrchg]

Summary

• Keep historical chat diffs rendered as the same inspectable diff cards instead of compacting them into Earlier change summaries.
• Remove the Recent change presentation path and the two-cycle recency cutoff from the chat timeline.
• Update chat message/list tests to cover the new presentation invariant.

Why

The old split between recent and earlier diff cards was purely presentational. It made older changes harder to inspect even though the chat diff cards are not the review/accept/reject surface. This keeps the timeline consistent while leaving the review, hunks, accept/reject, and buffer flows untouched.

Validation

• npm --prefix apps/desktop test -- src/features/ai/components/AIChatMessageItem.test.tsx src/features/ai/components/AIChatMessageList.test.tsx

[jsgrrchg]

Initially this logic was implemented to keep the chat streaming performant, but during testing, I found that it was not needed. Now users will be able to inspect every diff in the chat streaming no matter how old it is.