Add failing tests for the project name and build context

script/projectname still prints the pre-rename name, so make docker tags
its image quack, and .dockerignore has drifted from .gitignore: a compiled
bin/quak, the vitest and tsc caches, the CLI's runtime directory and the
local worktree directory all reach the build context. Neither failure is
visible in a build that exits 0.

Red until the fixes land.

test/packaging/build-context.test.ts

@@ -0,0 +1,49 @@
+// The Docker build context is load-bearing in two directions, and both
+// failures are silent.
+//
+// Excluding too little: a worktree left under `.claude/` is copied into the
+// image, vitest globs its `test/` tree as well as the real one, and the
+// containerised `make check` runs the whole suite twice over while reporting
+// success. A compiled `bin/quak` is ~100 MB of context nobody needs.
+//
+// Excluding too much: Prettier 3 reads `.gitignore` as a default ignore file,
+// so dropping it from the context silently changes which files
+// `make fmt-check` looks at inside the image compared to the host.
+//
+// Neither shows up as a build failure, so they are asserted here.
+import { describe, expect, it } from "vitest";
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { join } from "node:path";
+
+const repoRoot = fileURLToPath(new URL("../../", import.meta.url));
+
+const patterns = (name: string): string[] =>
+    readFileSync(join(repoRoot, name), "utf-8")
+        .split("\n")
+        .map((line) => line.trim())
+        .filter((line) => line !== "" && !line.startsWith("#"));
+
+const dockerignore = patterns(".dockerignore");
+
+describe(".dockerignore", () => {
+    // Everything here is either generated, enormous, or secret. `.claude/` is
+    // the correctness one: see the header comment and issue #25.
+    it.each([
+        ".claude/",
+        ".quak/",
+        "bin/quak",
+        "node_modules",
+        "coverage",
+        "dist",
+        ".vitest-cache/",
+        ".nyc_output/",
+        "*.tsbuildinfo",
+    ])("keeps %s out of the build context", (pattern) => {
+        expect(dockerignore).toContain(pattern);
+    });
+
+    it("leaves .gitignore in the build context for prettier", () => {
+        expect(dockerignore).not.toContain(".gitignore");
+    });
+});