docs: finalize README, attribute fork, sync AGENTS.md

- Rewrite README for fork context, add changes-from-upstream section,
  prune sections duplicated by docs/, surface AppImage as preferred run
- Add fork maintainer alongside upstream author in pyproject.toml
- Sync AGENTS.md with completed patch and deliberate supply-chain decisions

AGENTS.md

@@ -14,12 +14,15 @@ Vannini). Runs on Linux over Wayland or X11.
 This repo is a **local fork** of
 [macrodev8/ScanSlicer](https://github.com/macrodev8/ScanSlicer) (upstream owner
 inactive). License: GNU GPL v3 (must be preserved). Attribution: keep upstream
-credit and the "inspired by PhotoSlicer" note.
+credit and the "inspired by PhotoSlicer" note; `pyproject.toml` lists both the
+fork maintainer and the upstream author, and `README.md` states the fork
+relationship and the changes made.
 
 ## Goals of this fork
 
 1. Fix the mirrored-export orientation bug and add an explicit left/right
-   orientation workflow (implemented, see below). This is the primary patch.
+   orientation workflow. Implemented, tested in the GUI, and committed — see
+   "The orientation fix" below. This was the primary patch.
 2. Keep the app usable on the maintainer's own photo scans; afterwards only
    minimal maintenance is expected.
 3. Push to the maintainer's Forgejo instance at `code.dgerdesmann.de` (no
@@ -114,6 +117,12 @@ uv run scanslicer /path/to/scans     # or: python -m scanslicer
 - Dependencies (from `pyproject.toml`): PySide6, opencv-python-headless, numpy.
 - Python >= 3.10; dev env is Python 3.12 in `.venv`.
 
+Dependency / supply-chain decisions (deliberate — keep them):
+- `opencv-python-headless` is intentional: the app uses no OpenCV GUI/highgui.
+- Pillow was removed (it was unused). Don't re-add without a concrete need.
+- The AppImage build exports hashes from `uv.lock` and installs with
+  `pip install --require-hashes`.
+
 ### AppImage build: Docker context gotcha
 
 The maintainer's Docker points at Docker Desktop (`desktop-linux` context). Its