Add use_cases test category as Claude Code skill instructions

Use cases are markdown files that Claude Code reads, executes step by step
using its tools, and evaluates with its own judgment — not assertion scripts.

- cli_startup.md: pipe EOF into cli.py, verify banner and exit code 0
- apple_pie_research.md: /think query → complex tier → web_search + fetch →
  evaluate recipe quality, sources, and structure

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

tests/use_cases/cli_startup.md

@@ -0,0 +1,18 @@
+# Use Case: CLI Startup
+
+Verify the Adolf CLI starts cleanly and exits without error when the user closes input.
+
+## Steps
+
+Run the CLI with empty stdin (simulates user pressing Ctrl+D immediately):
+
+```bash
+echo "" | python3 /home/alvis/adolf/cli.py --session use-case-cli-startup
+echo "exit code: $?"
+```
+
+## Pass if
+
+- Output contains `Adolf CLI`
+- Output contains the session name and gateway URL
+- Exit code is 0