perf(ci): add sharded test runner phases

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

script/run-ci-tests.test.ts

@@ -1,4 +1,5 @@
 import { describe, expect, test } from "bun:test"
+import { selectCiTestTargets } from "./run-ci-tests"
 
 describe("test script isolation", () => {
   test("#given mock.module tests in the suite #then bun run test uses the isolated CI runner", async () => {
@@ -8,4 +9,37 @@ describe("test script isolation", () => {
     //#then
     expect(packageJson.scripts.test).toBe("bun run script/run-ci-tests.ts")
   })
+
+  test("#given isolated test shards #when selecting targets #then shards are deterministic and complete", () => {
+    // given
+    const ciTestPlan = {
+      isolatedModuleMockFiles: [],
+      isolatedTestTargets: ["a.test.ts", "b.test.ts", "c.test.ts", "d.test.ts", "e.test.ts"],
+      sharedTestFiles: ["shared.test.ts"],
+    }
+
+    // when
+    const shardOne = selectCiTestTargets(ciTestPlan, { phase: "isolated", shardCount: 2, shardIndex: 0 })
+    const shardTwo = selectCiTestTargets(ciTestPlan, { phase: "isolated", shardCount: 2, shardIndex: 1 })
+
+    // then
+    expect(shardOne).toEqual({ isolatedTestTargets: ["a.test.ts", "c.test.ts", "e.test.ts"], sharedTestFiles: [] })
+    expect(shardTwo).toEqual({ isolatedTestTargets: ["b.test.ts", "d.test.ts"], sharedTestFiles: [] })
+    expect([...shardOne.isolatedTestTargets, ...shardTwo.isolatedTestTargets].sort()).toEqual(ciTestPlan.isolatedTestTargets)
+  })
+
+  test("#given shared phase #when selecting targets #then only shared tests run", () => {
+    // given
+    const ciTestPlan = {
+      isolatedModuleMockFiles: [],
+      isolatedTestTargets: ["isolated.test.ts"],
+      sharedTestFiles: ["shared.test.ts"],
+    }
+
+    // when
+    const selectedTargets = selectCiTestTargets(ciTestPlan, { phase: "shared", shardCount: 1, shardIndex: 0 })
+
+    // then
+    expect(selectedTargets).toEqual({ isolatedTestTargets: [], sharedTestFiles: ["shared.test.ts"] })
+  })
 })