feat(frontend): set up Vitest frontend testing infrastructure with CI workflow (#2147)

* feat: set up Vitest frontend testing infrastructure with CI workflow

Migrate existing 4 frontend test files from Node.js native test runner
(node:test + node:assert/strict) to Vitest, reorganize test directory
structure under tests/unit/ mirroring src/ layout, and add a dedicated
CI workflow for frontend unit tests.

- Add vitest as devDependency, remove tsx
- Create vitest.config.ts with @/ path alias
- Migrate tests to Vitest API (test/expect/vi)
- Rename .mjs test files to .ts
- Move tests from src/ to tests/unit/ (mirrors src/ layout)
- Add frontend/Makefile `test` target
- Add .github/workflows/frontend-unit-tests.yml (parallel to backend)
- Update CONTRIBUTING.md, README.md, AGENTS.md, CLAUDE.md

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* style: fix the lint error

* style: fix the lint error

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>

frontend/src/core/threads/utils.test.ts

@@ -1,54 +0,0 @@
-import assert from "node:assert/strict";
-import test from "node:test";
-
-const { pathOfThread } = await import(
-  new URL("./utils.ts", import.meta.url).href
-);
-
-void test("uses standard chat route when thread has no agent context", () => {
-  assert.equal(pathOfThread("thread-123"), "/workspace/chats/thread-123");
-  assert.equal(
-    pathOfThread({
-      thread_id: "thread-123",
-    }),
-    "/workspace/chats/thread-123",
-  );
-});
-
-void test("uses agent chat route when thread context has agent_name", () => {
-  assert.equal(
-    pathOfThread({
-      thread_id: "thread-123",
-      context: { agent_name: "researcher" },
-    }),
-    "/workspace/agents/researcher/chats/thread-123",
-  );
-});
-
-void test("uses provided context when pathOfThread is called with a thread id", () => {
-  assert.equal(
-    pathOfThread("thread-123", { agent_name: "ops agent" }),
-    "/workspace/agents/ops%20agent/chats/thread-123",
-  );
-});
-
-void test("uses agent chat route when thread metadata has agent_name", () => {
-  assert.equal(
-    pathOfThread({
-      thread_id: "thread-456",
-      metadata: { agent_name: "coder" },
-    }),
-    "/workspace/agents/coder/chats/thread-456",
-  );
-});
-
-void test("prefers context.agent_name over metadata.agent_name", () => {
-  assert.equal(
-    pathOfThread({
-      thread_id: "thread-789",
-      context: { agent_name: "from-context" },
-      metadata: { agent_name: "from-metadata" },
-    }),
-    "/workspace/agents/from-context/chats/thread-789",
-  );
-});