After splitting from the original scl_lsp repo, update documentation
and test infrastructure to reflect the server-only scope of this repo.
- README.md: focused on LSP server features, tree-sitter grammar,
running the server, and Mason installation reference
- AGENTS.md: server-only project structure and conventions
- test/run_tests.lua: run only server tests (formatter, plc_json)
- Add comprehensive formatter test suite with 38 tests
- Test multiline assignments, FB calls, control structures
- Test IF, FOR, WHILE, CASE, REPEAT statements
- Test regions, structs, var sections
- Update AGENTS.md with formatter test command