fix: propagate exit codes in youtube-summarizer --list mode

Fixed automation-breaking issue where --list mode always returned exit code 0,
even when list_available_transcripts() failed due to invalid video ID or network errors.

Changes:
- extract-transcript.py: Capture return value and exit with proper status code
  - Before: list_available_transcripts(video_id); sys.exit(0)
  - After: success = list_available_transcripts(video_id); sys.exit(0 if success else 1)
- SKILL.md: Bumped version to 1.2.1
- CHANGELOG.md: Created changelog with v1.2.1 release notes

Impact: Automation scripts can now detect failures correctly via exit codes.

Identified by Codex automated review in antigravity-awesome-skills PR #62.

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

skills/youtube-summarizer/scripts/extract-transcript.py

@@ -56,8 +56,8 @@ if __name__ == "__main__":
     
     # Check if user wants to list available transcripts
     if len(sys.argv) > 2 and sys.argv[2] == "--list":
-        list_available_transcripts(video_id)
-        sys.exit(0)
+        success = list_available_transcripts(video_id)
+        sys.exit(0 if success else 1)
     
     # Extract transcript
     language = sys.argv[2] if len(sys.argv) > 2 else 'en'