Merge feature/streaming into development

Stream assistant tokens end to end: on-device (LocalBackend via onde) and over the cloud (OpenAiBackend via SSE), surfaced live in both the TUI and ACP. Adds the run-sigit skill to build and drive the binary. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> # Conflicts: # src/chat.rs