Split plugin development guide from root CLAUDE.md into module-level files (#790)

Move plugin development content (V2/V1 APIs, class matching, tracing concepts,
meter APIs, plugin test framework, etc.) into dedicated CLAUDE.md files under
apm-sniffer/apm-sdk-plugin/ and apm-sniffer/bootstrap-plugins/, reducing the
root CLAUDE.md from ~700 lines to ~300 lines for better maintainability.

Author: wu-sheng