feat: Save new discussions in discussions/ subdirectory

When creating a discussion without an explicit output path, files are now
saved to discussions/ subdirectory instead of current directory. The
directory is created automatically if it doesn't exist.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/discussions/cli.py

@@ -27,7 +27,10 @@ def cmd_new(args) -> int:
     else:
         slug = args.title.lower().replace(" ", "-")
         slug = "".join(c for c in slug if c.isalnum() or c == "-")
-        path = Path(f"{slug}.discussion.md")
+        path = Path("discussions") / f"{slug}.discussion.md"
+
+    # Ensure parent directory exists
+    path.parent.mkdir(parents=True, exist_ok=True)
 
     if path.exists() and not args.force:
         print(f"Error: {path} already exists. Use --force to overwrite.")