fix: add @app.callback so codex --help exits 0

Bare Typer() with no commands raised RuntimeError at runtime.
Callback makes the entry point callable without exposing F-07 logic.

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

codex/cli.py

@@ -1,3 +1,8 @@
 import typer
 
 app = typer.Typer(help="codex — personal knowledge base for scientific papers.")
+
+
+@app.callback()
+def _main() -> None:
+    """codex CLI — commands land here in F-07."""