Add plugin registry example package

[weich97]

Scope

Add one curated plugin example under plugins/examples/ and document how it differs from generated local scaffolds.

Suggested files

• plugins/examples/
• plugins/README.md
• docs/plugin_development.md
• tests/

Acceptance criteria

• Example imports cleanly.
• Example has one deterministic test.
• README explains when to start from tradearena new-plugin versus the curated example.

Validation

python -m pytest tests/test_plugin_examples.py -q