feat: add --debug flag with levels and centralized logging

CHANGES
- Add --debug flag controlling runtime logging verbosity levels
- Introduce internal/log package with Off, Basic, Detailed, Trace
- Replace ad-hoc Debugf and globals with centralized debug logger
- Wire debug level during early CLI argument parsing
- Add bash, zsh, fish completions for --debug levels
- Document debug levels in README with usage examples
- Add comprehensive STT guide covering models, flags, workflows
- Simplify splitAudioFile signature and log ffmpeg chunking operations
- Remove FABRIC_STT_DEBUG environment variable and related code
- Clean minor code paths in vendors and template modules

README.md

@@ -636,10 +636,20 @@ Application Options:
       --yt-dlp-args=                Additional arguments to pass to yt-dlp (e.g. '--cookies-from-browser brave')
       --thinking=                   Set reasoning/thinking level (e.g., off, low, medium, high, or
                                     numeric tokens for Anthropic or Google Gemini)
+      --debug=                     Set debug level (0: off, 1: basic, 2: detailed, 3: trace)
 Help Options:
   -h, --help                        Show this help message
 ```
 
+### Debug Levels
+
+Use the `--debug` flag to control runtime logging:
+
+- `0`: off (default)
+- `1`: basic debug info
+- `2`: detailed debugging
+- `3`: trace level
+
 ## Our approach to prompting
 
 Fabric _Patterns_ are different than most prompts you'll see.