fix(reddit): use removeprefix("r/") for subreddit names, not lstrip("r/")

str.lstrip("r/") treats its argument as a character set, stripping
leading r and / repeatedly. Subreddits starting with 'r' (e.g. r/robotics,
r/ruby) were silently mangled to 'obotics' / 'uby'. Replace with
str.removeprefix("r/") at all four call sites. Python 3.9+ pattern is
safe here — project requires 3.12. Closes #288.

Co-authored-by: Alex Key <alexanderkey0508@gmail.com>

skills/last30days/scripts/lib/entity_extract.py

@@ -106,7 +106,7 @@ def _extract_subreddits(reddit_items: List[Dict[str, Any]]) -> List[str]:
 
     for item in reddit_items:
         # Primary subreddit
-        sub = item.get("subreddit", "").strip().lstrip("r/")
+        sub = item.get("subreddit", "").strip().removeprefix("r/")
         if sub:
             sub_counts[sub] += 1