opencode custom LSP integration guide and smoke tests

[michaeldistel]

Summary

Add a documented OpenCode custom LSP path for the existing Structured Text server.

Scope

• Document opencode.json config for .st and .iecst
• Add a stdio server entrypoint suitable for OpenCode custom LSP launch
• Validate diagnostics, completion, definition, and hover on sample files
• Keep VS Code extension behaviour unchanged

Implementation Notes

• Keep server host editor-agnostic (no VS Code-only startup assumptions)
• Reuse existing providers and tests where possible
• Add OpenCode setup notes under README or docs

Acceptance

• OpenCode launches the server from custom LSP config
• Diagnostics and completion work on sample ST files
• Go to definition and hover resolve expected symbols
• npm run test:unit and npm run webpack-prod pass