Remove active Homeboy notification command shim

[chubes4]

Summary

• remove the OpenCode plugin that rewrote every homeboy command
• remove the wp-coding-agents-homeboy-notification wrapper
• scrub stale wrapper, plugin, and opencode.json entries during setup and upgrades
• retain passive asynchronous notification ownership in Homeboy and its extensions

Verification

• for test_file in tests/*.sh; do bash "$test_file"; done
• node tests/effective-prompt/run.mjs
• git diff --check
• final identifier audit: remaining names are limited to migration cleanup/tests and the historical changelog entry

Closes #300.

AI assistance

OpenCode using openai/gpt-5.6-sol investigated the regression, implemented the removal through a general coding task, and assisted with review and verification. Chris Huber remains responsible for every line.