Add a section explaining how to diagnose and fix packages stuck in
DependenciesNotReady status, which commonly happens when a required
package in the dependency chain is disabled.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Andrei Kvapil <kvapss@gmail.com>

Update `make image-cozystack` to `make image-packages` to match the
current Makefile target.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Andrei Kvapil <kvapss@gmail.com>