feat: Migrate to SARIF as the intermediate format

- Remove JSON v2 document model and related tooling
- Add SARIF output generation for documentation model
- Add SARIF reader for parsing SARIF back into internal types
- Update `stig generate` command to produce SARIF output by default
- Update `stig render` command to consume SARIF input
- Simplify CLI arguments for `generate` command by removing output format and watch/serve options
- Remove preprocessor and callgraph modules
- Remove filewatcher and incremental cache modules for simplification
- Update README to reflect the new SARIF-based pipeline and commands
- Streamline architecture by consolidating intermediate representation

Author: bresilla