Implement 'dws plugin config set/get/list/unset' commands that persist
plugin configuration (e.g. API keys) to ~/.dws/settings.json. Values
are automatically injected as environment variables at plugin load time,
so ${KEY} references in plugin.json headers/endpoints resolve without
manual 'export' each session.
Key changes:
- loader.go: add Get/Set/Unset/List/InjectPluginConfigEnv methods
- plugin_cmd.go: add 'plugin config' subcommand group (set/get/list/unset)
- root.go: call InjectPluginConfigEnv() before plugin loading
- loader_config_test.go: 9 unit tests covering all new functionality
User env vars take precedence over settings.json values.
Sensitive values are masked in 'plugin config list' output.
