Add viewMode option to hide diffs only (keep other tool output)

Current state

viewMode has 3 options:

• default — shows everything (including big diffs that fill the screen)
• verbose — even more detail
• focus — hides ALL tool calls + results (too aggressive, loses important context like Read/Bash output)

Use case

For routine code edits, I trust the assistant and don't need to see the full diff blob in the chat transcript (I can review via git diff or my editor). But I DO want to see other tool calls (Read, Bash, Grep) and the assistant's reasoning text.

Proposed

A new viewMode value (e.g. "compact" / "no-diffs"), OR a separate setting like "hideEditPreviews": true, that:

• Hides Edit/Write/NotebookEdit result diffs (or replaces them with a one-line ✓ Edited file.py:L10-L30)
• Keeps all other tool calls/results visible
• Keeps assistant text fully visible

Why this matters

Large refactors generate enormous diff blocks that bury the conversation. Scrolling back to find earlier reasoning becomes painful. focus mode "solves" this by hiding too much; default is too noisy. A middle ground would significantly improve the transcript usability for routine edit-heavy work.