[sean0407]

Fixes #994.

A flag from a shared @OptionGroup is listed by a nested subcommand's help but was rejected when supplied after that subcommand's positional argument, if the root command had a passthrough defaultSubcommand.

When a child command has parsed values for an option group already decoded by its parent, use the child's parsed value and carry it forward to deeper commands. When the child has no value, keep the parent's decoded value. Passthrough commands continue reusing the parent value.

The regression test models a root shared --debug flag, nested image tag <name> commands, and a passthrough default plugin command.

Checks run in a network-disabled container with Swift 6.2.3 on Linux arm64:

• swift test (567 tests passed)
• swift-format lint --strict on the four changed files
• git diff --check