feat(FN-3234): refactor test-changed script and add workflow tests

This merge refactors the test-changed script and adds new CLI tests (FN-3234), keeping automation on the full-suite path. It includes a new test file for the root test command, updates to the CI workflow, and documentation improvements.

Fusion-Task-Id: FN-3234

packages/cli/src/__tests__/root-test-command.test.ts

@@ -0,0 +1,57 @@
+import { describe, expect, it } from "vitest";
+import {
+  decideExecutionPlan,
+  resolveAffectedPackages,
+  shouldForceFullSuite,
+} from "../../../../scripts/test-changed.mjs";
+
+describe("root test command changed-only planning", () => {
+  it("uses changed mode when package-only changes are detected", () => {
+    const packageMap = new Map([
+      ["core", "@fusion/core"],
+      ["engine", "@fusion/engine"],
+    ]);
+
+    const plan = decideExecutionPlan({
+      forceFullSuite: false,
+      comparisonBase: "abc123",
+      changedFiles: ["packages/core/src/store.ts", "packages/engine/src/index.ts"],
+      packageNameByDir: packageMap,
+    });
+
+    expect(plan).toEqual({ mode: "changed", packages: ["@fusion/core", "@fusion/engine"] });
+  });
+
+  it("falls back to full suite when shared test infra changes", () => {
+    const plan = decideExecutionPlan({
+      forceFullSuite: false,
+      comparisonBase: "abc123",
+      changedFiles: ["scripts/test-with-lock.mjs"],
+      packageNameByDir: new Map([["core", "@fusion/core"]]),
+    });
+
+    expect(plan).toEqual({ mode: "full", reason: "shared-infra-changed" });
+  });
+
+  it("falls back to full suite when comparison base cannot be resolved", () => {
+    const plan = decideExecutionPlan({
+      forceFullSuite: false,
+      comparisonBase: null,
+      changedFiles: null,
+      packageNameByDir: new Map(),
+    });
+
+    expect(plan).toEqual({ mode: "full", reason: "missing-comparison-base" });
+  });
+
+  it("treats unknown package directories as full-suite fallback", () => {
+    const resolved = resolveAffectedPackages(["packages/unknown/src/index.ts"], new Map());
+    expect(resolved).toBeNull();
+  });
+
+  it("marks root workflow/config changes as full-suite triggers", () => {
+    expect(shouldForceFullSuite([".github/workflows/ci.yml"])).toBe(true);
+    expect(shouldForceFullSuite(["package.json"])).toBe(true);
+    expect(shouldForceFullSuite(["packages/core/src/store.ts"])).toBe(false);
+  });
+});