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/CHANGELOG.md

@@ -0,0 +1,66 @@
+# Changelog - youtube-summarizer
+
+All notable changes to the youtube-summarizer skill will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+---
+
+## [1.2.1] - 2026-02-04
+
+### 🐛 Fixed
+
+- **Exit code propagation in `--list` mode**
+  - **Issue:** Script always exited with status 0 even when `list_available_transcripts()` failed
+  - **Risk:** Broke automation pipelines that rely on exit codes to detect failures
+  - **Root Cause:** Return value from `list_available_transcripts()` was ignored
+  - **Solution:** Now properly checks return value and exits with code 1 on failure
+  - **Impact:** Scripts in automation can now correctly detect when transcript listing fails (invalid video ID, network errors, etc.)
+
+### 🔧 Changed
+
+- `extract-transcript.py` (lines 58-60)
+  - Before: `list_available_transcripts(video_id); sys.exit(0)`
+  - After: `success = list_available_transcripts(video_id); sys.exit(0 if success else 1)`
+
+### 📝 Notes
+
+- **Breaking Change:** None - only affects error handling behavior
+- **Backward Compatibility:** Scripts that check exit codes will now work correctly
+- **Migration:** No changes needed for existing users
+
+### 🔗 Related
+
+- Identified by Codex automated review in antigravity-awesome-skills PR #62
+- Also fixed in antigravity-awesome-skills fork
+
+---
+
+## [1.2.0] - 2026-02-04
+
+### ✨ Added
+
+- Intelligent prompt workflow integration
+- LLM processing with Claude CLI or GitHub Copilot CLI
+- Progress indicators with rich terminal UI
+- Multiple output formats
+- Enhanced error handling
+
+### 🔧 Changed
+
+- Major refactor of transcript extraction logic
+- Improved documentation in SKILL.md
+- Updated installation requirements
+
+---
+
+## [1.0.0] - 2025-02-01
+
+### ✨ Initial Release
+
+- YouTube transcript extraction
+- Language detection and selection
+- Basic summarization
+- Markdown output format
+- Support for multiple languages