feat: validate plugin config examples with matrix

[joy7758]

Summary

• add a second tracked plugin config example that covers the package-name-plus-package-path variant
• split plugin config validation into its own matrix job so each tracked example is checked independently without duplicating the rest of the validation job
• update the README to explain what the second example demonstrates

Validation

• python3 validation/scripts/validate_plugin_config.py plugin_config.example.json
• python3 validation/scripts/validate_plugin_config.py plugin_config.packages.example.json
• python3 demos/persona_workflow_demo.py
• POP_PLUGIN_CONFIG_FILE=/tmp/pop_demo_plugins/plugin_config.json python3 demos/persona_workflow_demo.py --task-input /tmp/content_strategy_task.json