Split the docs: README for users, DEVELOPMENT.md for developers

The README now explains setup, workflows, the mail options with Deck's
real menu names, and what happens when boards or stacks disappear -
without class names. Architecture, data model, tests, CI, release and
the time_sensitive troubleshooting move to DEVELOPMENT.md, fixing the
stale CI table, PHP version and Nextcloud version on the way.
DEVELOPMENT.md is excluded from the app package.

Makefile

@@ -24,6 +24,7 @@ appstore: clean
 		--exclude=./.php-cs-fixer.cache \
 		--exclude=./phpunit.xml \
 		--exclude=./README.md \
+		--exclude=./DEVELOPMENT.md \
 		--exclude=./Makefile \
 		--exclude=./CLAUDE.md \
 		--exclude=./CLAUDE.local.md \