feat: add -b/--brief flag for compact list output (#505)

Add a --brief flag that lists cheatsheets with only title and tags,
omitting the file path column. Can be used standalone (-b) or combined
with --list (-lb), and composes with existing filters (-t, -p).

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

README.md

@@ -70,6 +70,12 @@ To list all available cheatsheets:
 cheat -l
 ```
 
+To briefly list all cheatsheets (names and tags only):
+
+```sh
+cheat -b
+```
+
 To list all cheatsheets that are tagged with "networking":
 
 ```sh