Make --list-enabled mutually exclusive with enable/disable subcommands

[Copilot]

The --list-enabled flag was silently ignored when combined with enable/disable subcommands due to conditional logic that only checked the flag when no subcommands were present.

Changes

• Added yargs conflicts declarations to make --list-enabled mutually exclusive with enable/disable subcommands
• Fixed missing spread operator in disablePlugin that would overwrite entire config instead of preserving fields
• Added defensive null check for enabledPlugins array

Behavior

# Now produces clear error instead of silently ignoring flag
$ construct plugin enable foo --list-enabled
Arguments pluginName and list-enabled are mutually exclusive

# List command works independently
$ construct plugin --list-enabled
No plugins enabled.

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