docs(fold-extra-into-apps): add "fold extra into apps" design proposal

Retire the packages/extra bucket by folding those packages into apps and
expressing hidden-from-catalog / per-tenant-singleton / shared-down-the-tree
as declarative capabilities on ApplicationDefinition (visibility, cardinality,
protection, capability with share).

Supersedes community#4 (tenant module overrides); complementary to
community#25 (per-cluster etcd). Grounded against cozystack main @ cece5c23b.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Myasnikov Daniil <myasnikovdaniil2001@gmail.com>

Author: myasnikovdaniil