[Feature Request] Zero-height tool renders — suppress spacer/container when extension returns empty component

[Joshua-37S]

Summary

When a custom tool extension returns an empty/nil component from renderCall and renderResult, pi still renders a visible gap for each tool call. This makes it impossible to fully hide tool activity via the extension API.

Root cause

In packages/coding-agent/src/modes/interactive/components/tool-execution.ts, the ToolExecutionComponent constructor unconditionally adds spacing regardless of what the extension returns:

constructor(toolName, args, options, toolDefinition, ui, cwd) {
    super();
    this.addChild(new Spacer(1));          // always added
    this.contentBox = new Box(1, 1, bgFn); // paddingY=1 top+bottom
    this.addChild(this.contentBox);

renderCall and renderResult only control what goes inside contentBox. The Spacer(1) above and Box(paddingY=1) wrapper are unconditional. Each hidden tool call still contributes ~3 blank lines to the layout.

Reproduction

1. Install the bundled minimal-mode.ts example extension (or any extension that returns { render: () => [], invalidate: () => {} } from both renderCall and renderResult)
2. Run a response that triggers 5+ parallel tool calls
3. Observe a proportional blank gap between the user message and the assistant response — even though no tool output is visible

Expected behaviour

When renderCall returns a zero-height component (i.e. render() returns []), and renderResult does the same, the tool execution container — including its leading Spacer and Box padding — should contribute zero lines to the layout. The assistant response should flow seamlessly from the previous message with no gap.

Proposed fix

Option A — skip the spacer and container when rendered output is empty (no API change required):

const callLines = callComponent.render(width);
const resultLines = resultComponent?.render(width) ?? [];
if (callLines.length + resultLines.length === 0) {
    return; // render nothing — skip spacer and container entirely
}

Option B — let extensions opt out by returning null from renderCall:

renderCall?: (args, theme) => Component | null;
// null = do not render any wrapper for this tool call

Option C — add a hideWhenEmpty?: boolean flag to registerTool:

pi.registerTool({
    name: "bash",
    hideWhenEmpty: true,
    renderCall() { return nil; },
    renderResult() { return nil; },
    ...
})

Context

• pi version: 0.55.3
• Use case: the bundled minimal-mode.ts / silent mode extension, where the goal is to hide all tool activity unless the user explicitly expands via Ctrl+O. The gap breaks the seamless feel the extension is designed to achieve.
• Option A is the cleanest fix — purely internal to tool-execution.ts, no API surface changes required.