Split README into a focused user guide + dedicated DeveloperGuide.md

The README had grown to mix user-facing setup with internal CLI details,
file-layout internals, and build commands — too dense for someone picking
up AutoNote for the first time. Rewrite README.md as a tight user guide
covering: quick start, the seven pages, Pipeline + Generate options
(including the new image-source and backend dropdowns), where files end
up, language switching, and troubleshooting. The image-source guidance
also explains the v0.13.5 behaviour: picking "Video screenshots" forces
frame extraction so picture-in-picture lectures still produce images.

Move repository layout, per-script CLI surface, multi-provider LLM
dispatch, the v0.13.6 classifier decision rule, the Panopto split-stream
merge, cache layouts, Electron app structure, test workflow, and the
release procedure into DeveloperGuide.md so contributors have one place
to find them.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: nodeeeeee