feat: infer Knip Addons from disk (#2086)

## PR Checklist

- [x] Addresses an existing open issue: fixes #2085
- [x] That issue was marked as [`status: accepting
prs`](https://github.com/JoshuaKGoldberg/create-typescript-app/issues?q=is%3Aopen+is%3Aissue+label%3A%22status%3A+accepting+prs%22)
- [x] Steps in
[CONTRIBUTING.md](https://github.com/JoshuaKGoldberg/create-typescript-app/blob/main/.github/CONTRIBUTING.md)
were taken

## Overview

🎁

Author: JoshuaKGoldberg

src/blocks/blockKnip.test.ts

@@ -1,5 +1,5 @@
-import { testBlock } from "bingo-stratum-testers";
-import { describe, expect, test, vi } from "vitest";
+import { testBlock, testIntake } from "bingo-stratum-testers";
+import { describe, expect, it, test, vi } from "vitest";
 
 import { blockKnip } from "./blockKnip.js";
 import { optionsBase } from "./options.fakes.js";
@@ -206,4 +206,36 @@ describe("blockKnip", () => {
 			}
 		`);
 	});
+
+	describe("intake", () => {
+		it("returns undefined when knip.json does not exist", () => {
+			const actual = testIntake(blockKnip, {
+				files: {},
+			});
+
+			expect(actual).toBeUndefined();
+		});
+
+		it("returns undefined when knip.json does not contain ignoreDependencies", () => {
+			const actual = testIntake(blockKnip, {
+				files: {
+					"knip.json": [JSON.stringify({ other: true })],
+				},
+			});
+
+			expect(actual).toBeUndefined();
+		});
+
+		it("returns ignoreDependencies when knip.json contains ignoreDependencies", () => {
+			const ignoreDependencies = ["a", "b", "c"];
+
+			const actual = testIntake(blockKnip, {
+				files: {
+					"knip.json": [JSON.stringify({ ignoreDependencies })],
+				},
+			});
+
+			expect(actual).toEqual({ ignoreDependencies });
+		});
+	});
 });

src/blocks/blockKnip.ts

@@ -10,13 +10,28 @@ import { blockGitHubActionsCI } from "./blockGitHubActionsCI.js";
 import { blockPackageJson } from "./blockPackageJson.js";
 import { blockRemoveFiles } from "./blockRemoveFiles.js";
 import { blockRemoveWorkflows } from "./blockRemoveWorkflows.js";
+import { intakeFileAsJson } from "./intake/intakeFileAsJson.js";
+
+const zIgnoreDependencies = z.array(z.string());
 
 export const blockKnip = base.createBlock({
 	about: {
 		name: "Knip",
 	},
 	addons: {
-		ignoreDependencies: z.array(z.string()).optional(),
+		ignoreDependencies: zIgnoreDependencies.optional(),
+	},
+	intake({ files }) {
+		const knipJson = intakeFileAsJson(files, ["knip.json"]);
+		if (!knipJson?.ignoreDependencies) {
+			return undefined;
+		}
+
+		return {
+			ignoreDependencies: zIgnoreDependencies.safeParse(
+				knipJson.ignoreDependencies,
+			).data,
+		};
 	},
 	produce({ addons }) {
 		const { ignoreDependencies } = addons;

src/blocks/intake/intakeFileAsJson.test.ts

@@ -0,0 +1,35 @@
+import { describe, expect, it } from "vitest";
+
+import { intakeFileAsJson } from "./intakeFileAsJson.js";
+
+describe(intakeFileAsJson, () => {
+	it("returns undefined when the file does not exist", () => {
+		const actual = intakeFileAsJson({}, []);
+
+		expect(actual).toBeUndefined();
+	});
+
+	it("returns undefined when the file does not have valid JSON", () => {
+		const actual = intakeFileAsJson(
+			{
+				"file.json": ["{"],
+			},
+			["file.json"],
+		);
+
+		expect(actual).toBeUndefined();
+	});
+
+	it("returns loaded file contents when the file has valid JSON", () => {
+		const value = { key: "value" };
+
+		const actual = intakeFileAsJson(
+			{
+				"file.json": [JSON.stringify(value)],
+			},
+			["file.json"],
+		);
+
+		expect(actual).toEqual(value);
+	});
+});

src/blocks/intake/intakeFileAsJson.ts

@@ -0,0 +1,14 @@
+import { IntakeDirectory } from "bingo-fs";
+import JSON5 from "json5";
+
+import { intakeFile } from "./intakeFile.js";
+
+export function intakeFileAsJson(files: IntakeDirectory, filePath: string[]) {
+	const file = intakeFile(files, filePath);
+
+	try {
+		return file && JSON5.parse<Record<string, unknown> | undefined>(file[0]);
+	} catch {
+		return undefined;
+	}
+}