Rewrite Solar homepage with SVG architecture overview

[lisihao]

Summary

• Rewrite the repository homepage around the current Solar architecture: Core, Harness, Knowledge, and autonomous software organization runtime.
• Replace Mermaid diagrams with maintained SVG architecture assets.
• Add three SVG diagrams:
  • docs/assets/solar-system-architecture.svg
  • docs/assets/solar-operator-dag.svg
  • docs/assets/solar-autonomous-loop.svg
• Clarify the role of Solar Harness as requirement compiler, agentic DAG runtime, physical/logical operator scheduler, evidence ABI, and model fleet control plane.
• Add current capability matrix, design principles, roadmap, and current-boundary section.

Notes

This PR intentionally does not modify runtime code. It only updates public-facing documentation and diagram assets.

Follow-up recommended

• Add a release/privacy scan gate for machine-local paths, host fingerprints, and runtime excerpts.
• Split local actor/physical-operator config into example vs local files.
• Align USER-GUIDE.md and install docs with the new homepage framing.