docs: document the sysext build as it stands

Write down the single-owner layout, the delivery contract and the
operating procedures while they are still current, so the design doc
becomes the single source of truth. The doc states the design as it
is: every invariant, gotcha and open item in Simplified Technical
English, no change log, no facts restated from tracked files.

Author: fwiesel

.gitignore

@@ -4,4 +4,5 @@
 shell.nix
 __pycache__/
 .pytest_cache/
+docs/sysext-handover.md
 .actrc