docs: add project README

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

README.md

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+# Runa — Rune Actors
+
+Runa (short for **Rune Actors**) is an experimental project to build a
+persistent, live programming world around isolated actors written in Rune.
+The long-term vision combines an inspectable graphical environment with a
+programmatic API and a VS Code extension, all connected to the same running
+image.
+
+## Status
+
+Runa is at an early prototype stage. The current repository contains a minimal
+Rust executable; the actor runtime, Rune integration, graphical world, and
+development API described in the architecture spec are planned, not yet
+implemented.
+
+The first milestone is to evaluate Rune's embedding, async execution,
+interruptibility, and debugging capabilities before building the actor kernel.
+
+## Design goals
+
+- Keep actor identity, mailbox, private state, behavior, and incarnation
+  separate.
+- Use message passing rather than shared mutable state for actor interaction.
+- Keep behavior implementations immutable and support safe, versioned updates.
+- Expose the same runtime capabilities to in-world tools and external clients.
+- Be explicit about execution, delivery, and persistence guarantees.
+
+See [runa-spec-architecture.md](runa-spec-architecture.md) for the full vision,
+architecture, roadmap, and open questions.
+
+## Getting started
+
+You need a Rust toolchain that supports the workspace's Rust 2024 edition.
+
+Run the current executable from the repository root:
+
+```sh
+cargo run -p runa
+```
+
+At this stage, it prints `Hello, world!`.
+
+## Project structure
+
+```text
+.
+├── Cargo.toml
+├── crates/
+│   └── runa/
+│       └── src/main.rs
+└── runa-spec-architecture.md
+```