Add Ctrl+J to toggle edit diffs independently of tool output

[snimu]

What this does

Adds Ctrl+J to show or hide edit diffs on their own, independent of Ctrl+O (tool output).

When the agent has made a few file edits in between many long tool calls, you can press Ctrl+J to see every edit's full diff — while all the noisy tool output stays collapsed. Ctrl+J is now the only switch for diffs: Ctrl+O still expands code and command output, but no longer shows or hides diffs. The two switches don't affect each other.

Why

Expanding tool output was the only way to see a diff in full. With long-running work that means wading through hundreds of lines of command output just to check a handful of edits.

How it works

• Every edit shows its one-line summary (path +N −M) with a dim (Ctrl+J to expand) hint next to it. With Ctrl+J on, the full diff renders in place of that summary — code and command output stay behind Ctrl+O.
• The switch follows the same pattern as the existing independent toggles (Ctrl+O tools, Ctrl+P agent messages, Ctrl+T thinking) and is remappable like every other keybinding.
• Default is off, and the choice isn't persisted between sessions, matching the other two expansion toggles.

Changes

• Diff rendering now keys off the new toggle alone, in both display paths: code cells (the usual case) and the classic built-in edit tool. The old "expand everything" overlap was removed, which simplified the display logic.
• Standard toggle plumbing: keybinding, hint texts, shortcuts panel, docs row.
• The edit summary line carries the Ctrl+J hint, following the existing hint style and only on the most recent tool, like other hints.
• Tests for the toggle in both paths, the hint, and the keybinding.

Lines changed (vs main): source +77/−11, tests +111/−9, docs and changelog +2/−0.

Checks

• npm run check clean; all touched test files pass (~260 tests across the areas).
• Validated end-to-end in a real terminal: 200-line noisy tool calls plus two edits — Ctrl+J shows just the diffs, off hides them, Ctrl+O expands code/output without touching diffs. Also verified Ctrl+J is received distinctly from Enter in tmux; terminals that can't send it can remap the binding.

Note

Add Ctrl+J keybinding to toggle edit diffs independently of tool output expansion

• Adds a new app.edits.expand keybinding (Ctrl+J) that toggles edit diff visibility independently of the general tool output expand/collapse toggle.
• Built-in edit tools and IPython-rendered cells now track a separate editDiffsExpanded state, so diffs can be shown while tool output remains collapsed (and vice versa).
• When diffs are collapsed, a FileChangeSummaryComponent shows a +N -M line count summary with a (Ctrl+J to expand/collapse) hint.
• The expansion state is propagated to all existing and newly created ToolExecutionComponent instances in InteractiveMode and reflected in the footer and help guide.
• Behavioral Change: diff visibility for built-in edit tools is no longer governed by the general tool expansion toggle — only Ctrl+J controls it.

^{Macroscope summarized 551c530.}

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