Add UAR OpenTUI Rust foundation

[GQAdonis]

What changed

Adds a self-contained uar-opentui/ monorepo foundation that pairs an OpenTUI/Solid terminal client with an authoritative Rust runtime process.

Included

• OpenCode-inspired transcript, composer, status bar, streaming behavior, and cancellation
• SolidJS + @opentui/solid TUI
• Rust JSON-lines IPC runtime over child-process stdio
• Shared Serde protocol crate with round-trip test
• streaming demo agent path that is ready to replace with the Universal Agent Runtime adapter
• Bun and Cargo workspace metadata
• CI for Rust formatting, Clippy, tests, and TypeScript checks
• architecture and run documentation

Why

Using OpenTUI directly provides the closest path to OpenCode-level terminal rendering and behavior while preserving Rust as the authority for agents, tools, permissions, sessions, MCP, A2A, and plugins.

Developer impact

Build the Rust runtime, install Bun dependencies, and launch the TUI from uar-opentui/. The UI/runtime boundary is already typed and streaming-capable.

Validation

The branch was structurally verified through the GitHub comparison API. Local compilation could not be run in the current execution environment because Rust and Bun are not installed there; CI is included to perform those checks on GitHub.