feat: add automatic OpenCode support

[SomSamantray]

Summary

OpenCode 1.18.29 or newer now adds Humanizer guidance automatically when run from this repository, with support for both V1 and V2 plugin hooks.

Closes #279.

Testing

• node --test scripts/test-opencode-plugin.mjs
• python3 scripts/validate-package.py
• npx --yes skills@1.5.20 add . --list
• claude plugin validate .
• OpenCode V2 CLI discovery smoke loaded the plugin; model execution stopped at the intentionally invalid model name before contacting a provider.

Post-Deploy Monitoring & Validation

No post-deploy action is needed. The plugin activates when OpenCode runs from this repository.

[blader]

Thanks for putting this together. As written, the plugin only loads when OpenCode runs inside this repository, so it does nothing for people using Humanizer on their own projects. It also injects the whole skill into every session, which I'd rather leave to each user. Closing. Users who want it always on can use the plugin snippet in #279.