fix(coding-agent): preserve indentation in rendered diffs (#9274)

## Problem

The edit tool's intra-line renderer can drop indentation from a removed
line when text is inserted before otherwise unchanged content.

For example, this input:

```diff
-83   await commitRecordingChunk({
+83   const result = await commitRecordingChunk({
```

was rendered as:

```diff
-83 await commitRecordingChunk({
+83   const result = await commitRecordingChunk({
```

`diffWords()` groups the shared leading spaces into the added segment
(`" const result = "`). The existing highlighting logic moved those
spaces outside inverse styling, but only appended them to the added
line.

## Solution

Extract whitespace shared by both lines before calling `diffWords()`,
then initialize both rendered lines with that prefix. This preserves
indentation without changing the existing word-level highlighting
behavior. The existing handling for whitespace that belongs to only one
side remains unchanged.

Added regression coverage for:

- insertion before indented content;
- replacement within indented content;
- keeping shared indentation outside inverse highlighting.

## Validation

- `npm run check`
- `./test.sh`
- targeted `packages/coding-agent/test/diff.test.ts` (3 tests)

packages/coding-agent/test/diff.test.ts

@@ -0,0 +1,35 @@
+import { beforeAll, describe, expect, it } from "vitest";
+import { renderDiff } from "../src/modes/interactive/components/diff.ts";
+import { initTheme, theme } from "../src/modes/interactive/theme/theme.ts";
+import { stripAnsi } from "../src/utils/ansi.ts";
+
+describe("renderDiff", () => {
+	beforeAll(() => initTheme("dark"));
+
+	it.each([
+		{
+			name: "text is inserted at the start of a line",
+			diff: "-83   await commitRecordingChunk({\n+83   const result = await commitRecordingChunk({",
+			expected: ["-83   await commitRecordingChunk({", "+83   const result = await commitRecordingChunk({"],
+		},
+		{
+			name: "indented text is replaced",
+			diff: "-12     const value = oldValue\n+12     const value = newValue",
+			expected: ["-12     const value = oldValue", "+12     const value = newValue"],
+		},
+	])("preserves indentation when $name", ({ diff, expected }) => {
+		const rendered = renderDiff(diff);
+
+		expect(stripAnsi(rendered).split("\n")).toEqual(expected);
+	});
+
+	it("does not highlight shared indentation as an insertion", () => {
+		const rendered = renderDiff(
+			"-83   await commitRecordingChunk({\n+83   const result = await commitRecordingChunk({",
+		);
+		const inverseInsertion = theme.inverse("const result = ");
+
+		expect(rendered.indexOf("+83   ")).toBeLessThan(rendered.indexOf(inverseInsertion));
+		expect(rendered).toContain(inverseInsertion);
+	});
+});