Add use_cases test category with CLI startup test

tests/use_cases/ holds scenario-driven tests run by the Claude Code agent,
which acts as both the test runner and mock user. Each test prints a
structured transcript; Claude evaluates correctness.

First test: test_cli_startup.py — spawns cli.py with a subprocess, reads
the welcome banner, sends EOF, and verifies exit code 0.

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

tests/use_cases/test_cli_startup.py

@@ -0,0 +1,46 @@
+#!/usr/bin/env python3
+"""
+Use case: CLI startup and clean exit.
+
+Starts the Adolf CLI, reads its welcome banner, then closes it by sending
+EOF (simulating Ctrl+D). Prints a structured transcript for the Claude Code
+agent to evaluate.
+
+Expected:
+  - Banner line contains "Adolf CLI"
+  - Prompt "> " appears
+  - Process exits with code 0 after EOF
+"""
+
+import os
+import subprocess
+import sys
+import time
+
+CLI = os.path.join(os.path.dirname(__file__), "../../cli.py")
+
+proc = subprocess.Popen(
+    [sys.executable, CLI, "--session", "use-case-cli-startup"],
+    stdin=subprocess.PIPE,
+    stdout=subprocess.PIPE,
+    stderr=subprocess.PIPE,
+    text=True,
+)
+
+# Give the process time to print its banner before closing stdin
+time.sleep(0.3)
+
+try:
+    stdout, stderr = proc.communicate(input="", timeout=10)
+except subprocess.TimeoutExpired:
+    proc.kill()
+    stdout, stderr = proc.communicate()
+    print("RESULT: TIMEOUT — process did not exit within 10s after EOF")
+    sys.exit(1)
+
+print("=== stdout ===")
+print(stdout)
+if stderr.strip():
+    print("=== stderr ===")
+    print(stderr)
+print(f"=== exit code: {proc.returncode} ===")