chore: developer ergonomics (lockfile, scripts, CI step, test mocks)

Bundles five pre-v0 ergonomics fixes that don't change runtime
behavior. Closes #2, #9, #10, #11, #12.

* `.gitignore` no longer ignores `bun.lock`; the lockfile is committed
  for reproducible installs.
* `package.json` gains a `test` script so `bun run test` works.
* `Makefile` gains a `fmt-check` target that runs the read-only
  `format:check` script; CI calls `make fmt-check` instead of writing
  files before `check_unstaged.sh`.
* `createMockResponse` in `src/test-helpers.ts` returns case-insensitive
  `Headers` instead of a `Map`. A regression test pins the contract.
* A Bun test preload (`bunfig.toml` + `src/setup-test.ts`) mocks
  `@actions/core` write helpers with no-ops so test output is readable.
  Existing per-test spies on `core.*` continue to work via `spyOn`.

🤖 Authored by Coder Agents.

Author: mafredri

.github/workflows/ci.yaml

@@ -28,7 +28,7 @@ jobs:
         run: make build
 
       - name: Format check
-        run: make fmt
+        run: make fmt-check
 
       - name: Check unstaged changes
         run: ./check_unstaged.sh

.gitignore

@@ -1,3 +1,2 @@
 node_modules/
-bun.lock
 .integration.tmp/

Makefile

@@ -1,4 +1,4 @@
-.PHONY: fmt lint test clean deps all
+.PHONY: fmt fmt-check lint test clean deps all
 
 TS_FILES := $(shell find src -name "*.ts" -type f ! -name "*.test.ts")
 
@@ -7,6 +7,9 @@ all: build fmt lint test
 fmt:
 	bun run format
 
+fmt-check:
+	bun run format:check
+
 lint:
 	bun run lint
 	bun run typecheck

bun.lock

@@ -0,0 +1,2 @@
+[test]
+preload = ["./src/setup-test.ts"]

bunfig.toml

@@ -9,7 +9,8 @@
 		"format": "biome format --write .",
 		"format:check": "biome format .",
 		"lint": "biome lint --error-on-warnings .",
-		"typecheck": "tsc --noEmit"
+		"typecheck": "tsc --noEmit",
+		"test": "bun test"
 	},
 	"dependencies": {
 		"@actions/core": "^1.10.1",

package.json

@@ -461,3 +461,17 @@ describe("CoderClient", () => {
 		});
 	});
 });
+
+describe("createMockResponse", () => {
+	test("headers behaves like fetch Response.headers (case-insensitive)", () => {
+		// Production code at coder-client.ts calls
+		// `response.headers?.get("content-length")` to detect empty bodies.
+		// The real `fetch` `Response.headers` is a case-insensitive `Headers`
+		// instance, so the mock must match. A regression to `new Map()` (the
+		// pre-fix behavior) would fail this assertion.
+		const res = createMockResponse({}, { headers: { "Content-Length": "0" } });
+
+		expect(res.headers.get("content-length")).toBe("0");
+		expect(res.headers.get("Content-Length")).toBe("0");
+	});
+});

src/coder-client.test.ts

@@ -0,0 +1,23 @@
+// Stub @actions/core during tests.
+//
+// The real write helpers (`info`, `warning`, `error`, etc.) emit GitHub
+// workflow commands (`::error::...`, `::warning::...`) and free-form
+// lines to stdout, which flood test output and obscure real failures.
+// Replace the used exports: write helpers become no-ops, and read helpers
+// return safe defaults so any test path that calls them stays
+// well-behaved. Tests that need to observe calls
+// (`spyOn(core, "warning")`) still work because `spyOn` wraps the
+// property on the mocked module.
+import { mock } from "bun:test";
+
+mock.module("@actions/core", () => ({
+	info: () => {},
+	debug: () => {},
+	warning: () => {},
+	error: () => {},
+	notice: () => {},
+	setFailed: () => {},
+	setOutput: () => {},
+	getInput: () => "",
+	getBooleanInput: () => false,
+}));

src/setup-test.ts

@@ -249,14 +249,19 @@ export function createMockOctokit() {
  */
 export function createMockResponse(
 	body: unknown,
-	options: { ok?: boolean; status?: number; statusText?: string } = {},
+	options: {
+		ok?: boolean;
+		status?: number;
+		statusText?: string;
+		headers?: Bun.HeadersInit;
+	} = {},
 ) {
 	return {
 		ok: options.ok ?? true,
 		status: options.status ?? 200,
 		statusText: options.statusText ?? "OK",
 		json: async () => body,
 		text: async () => JSON.stringify(body),
-		headers: new Map(),
+		headers: new Headers(options.headers),
 	};
 }