Major refactoring to unify runtime providers across sandbox and user script contexts:
1. Runtime Bridge & Router
- Add RuntimeMessageBridge for unified messaging abstraction
- Rename SandboxMessageRouter → RuntimeMessageRouter
- Router now handles both iframe and user script messages
- Guard for non-extension environments
2. Provider Refactoring
- ArtifactsRuntimeProvider: Add offline mode with snapshot fallback
- AttachmentsRuntimeProvider: Remove returnDownloadableFile (moved to dedicated provider)
- ConsoleRuntimeProvider: Add message collection, remove lifecycle logic
- FileDownloadRuntimeProvider: New provider for file downloads
3. HTML Escaping Fix
- Escape </script> in JSON.stringify output to prevent premature tag closure
- Applies when injecting provider data into <script> tags
- JavaScript engine automatically unescapes, no runtime changes needed
4. Function Renaming
- listFiles → listAttachments
- readTextFile → readTextAttachment
- readBinaryFile → readBinaryAttachment
- returnFile → returnDownloadableFile
5. Updated Exports
- Export new RuntimeMessageBridge and RuntimeMessageRouter
- Export FileDownloadRuntimeProvider
- Update all cross-references
This sets the foundation for reusing providers in browser-javascript tool.
