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/reddit_public.py

@@ -198,7 +198,7 @@ def search(
     encoded_query = _url_encode(query)
 
     if subreddit:
-        sub = subreddit.lstrip("r/").strip()
+        sub = subreddit.removeprefix("r/").strip()
         url = (
             f"https://www.reddit.com/r/{sub}/search.json"
             f"?q={encoded_query}&restrict_sr=on&sort=relevance&t=month&limit={limit}&raw_json=1"