docs: add project agent guidelines

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

AGENTS.md

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+# Project Guidelines
+
+## Project overview
+
+Runa (short for **Rune Actors**) is an experimental Rust project whose long-term
+goal is a persistent, live programming world built around isolated Rune actors.
+The current workspace is a minimal Rust executable; Rune integration and the
+actor runtime are not implemented yet.
+
+The architecture, design goals, constraints, and implementation roadmap are
+documented in [runa-spec-architecture.md](runa-spec-architecture.md). Treat
+proposed capabilities as plans, not existing behavior.
+
+## Architecture and constraints
+
+- Keep actor identity, mailbox, private state, behavior implementation, and
+  incarnation distinct as runtime work begins.
+- Use message passing for normal actor interaction; do not introduce shared
+  mutable actor state.
+- Keep behavior implementations immutable and install updates at safe
+  boundaries. Do not mutate active execution frames in place.
+- Start with the Rune feasibility work in Phase 0. Verify APIs and capabilities
+  against the actual Rune version before relying on them; do not claim
+  interruption, preemption, durability, or delivery guarantees without tests.
+- Build only the smallest workspace structure needed for the current milestone;
+  the crate layout in the spec is proposed, not a requirement to create empty
+  crates.
+
+## Repository structure
+
+- `Cargo.toml`: Rust workspace definition.
+- `crates/runa/`: current executable crate and entry point.
+- `runa-spec-architecture.md`: architecture, roadmap, risks, and acceptance
+  scenario.
+- `README.md`: project overview and current getting-started instructions.
+
+## Commands
+
+Run these from the repository root:
+
+```sh
+cargo fmt --all -- --check
+cargo check --workspace
+cargo clippy --workspace --all-targets -- -D warnings
+cargo test --workspace
+```
+
+The current project is a minimal executable and may not yet have tests.
+
+## Conventions
+
+- Keep changes focused and follow existing Rust formatting and workspace
+  conventions.
+- Prefer verified Rune APIs and explicit runtime guarantees over assumptions.
+- Keep the README and architecture spec aligned with implemented behavior;
+  label planned capabilities as planned.
+- Do not push changes unless explicitly asked.
+- Check the current branch before committing. Use conventional commit messages
+  and include the repository's configured Copilot co-author trailer.
+- Run relevant formatting, build, lint, and test checks before committing code
+  changes. Documentation-only changes do not require code checks.