feat: eval run command — prepare, execute, collect pipeline

Implement the uwf-eval run <task-dir> command with 3-phase pipeline:

- prepare: read task.yaml, copy fixture/ to temp workdir
- execute: shell out to uwf thread start + exec
- collect: run judges, compute weighted score, store CAS node,
  set @uwf/eval/<task>/latest variable

Changes:
- src/runner/ — types, prepare, execute, collect, index
- src/storage/store.ts — createEvalStore(), setEvalLatest()
- src/commands/run.ts — full pipeline wiring with --agent/--model/--count
- 9 new tests (prepare + collect + weighted scoring)

Builtin judges return placeholder score 0 (Phase 1c).

Refs #70

packages/eval/__tests__/prepare.test.ts

@@ -0,0 +1,74 @@
+import { mkdir, mkdtemp, readFile, rm, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+
+import { afterEach, beforeEach, describe, expect, test } from "vitest";
+
+import { prepare } from "../src/runner/index.js";
+
+const TASK_YAML = `
+name: fix-off-by-one
+description: Fix an off-by-one error
+workflow: solve-issue
+prompt: "Fix the bug"
+limits:
+  maxSteps: 12
+  timeoutMinutes: 20
+judges:
+  - name: frontmatter-compliance
+    weight: 0.5
+    builtin: true
+  - name: test-pass
+    weight: 0.5
+    entry: dist/judges/test-pass.js
+`;
+
+let taskDir: string;
+
+beforeEach(async () => {
+  taskDir = await mkdtemp(join(tmpdir(), "uwf-eval-task-"));
+  await writeFile(join(taskDir, "task.yaml"), TASK_YAML, "utf8");
+  const fixtureDir = join(taskDir, "fixture");
+  await mkdir(join(fixtureDir, "src"), { recursive: true });
+  await writeFile(join(fixtureDir, "src", "calc.ts"), "export const add = (a, b) => a + b + 1;\n");
+  await writeFile(join(fixtureDir, "package.json"), '{ "name": "fixture" }\n');
+});
+
+afterEach(async () => {
+  await rm(taskDir, { recursive: true, force: true });
+});
+
+describe("prepare", () => {
+  test("returns the parsed manifest", async () => {
+    const result = await prepare(taskDir);
+    expect(result.taskDir).toBe(taskDir);
+    expect(result.manifest.name).toBe("fix-off-by-one");
+    expect(result.manifest.workflow).toBe("solve-issue");
+    expect(result.manifest.limits.maxSteps).toBe(12);
+    expect(result.manifest.judges).toHaveLength(2);
+  });
+
+  test("copies fixture into a fresh temp work dir", async () => {
+    const result = await prepare(taskDir);
+    expect(result.workDir).not.toBe(taskDir);
+    expect(result.workDir.startsWith(tmpdir())).toBe(true);
+
+    const calc = await readFile(join(result.workDir, "src", "calc.ts"), "utf8");
+    expect(calc).toContain("export const add");
+    const pkg = await readFile(join(result.workDir, "package.json"), "utf8");
+    expect(pkg).toContain("fixture");
+
+    await rm(result.workDir, { recursive: true, force: true });
+  });
+
+  test("creates an empty work dir when no fixture/ exists", async () => {
+    const noFixtureDir = await mkdtemp(join(tmpdir(), "uwf-eval-nofix-"));
+    await writeFile(join(noFixtureDir, "task.yaml"), TASK_YAML, "utf8");
+
+    const result = await prepare(noFixtureDir);
+    expect(result.workDir.startsWith(tmpdir())).toBe(true);
+
+    await rm(noFixtureDir, { recursive: true, force: true });
+    await rm(result.workDir, { recursive: true, force: true });
+  });
+});