Task 5.3: Rust SDK

## What
Create `seren-store` Rust crate.

## Structure
```
src/
  lib.rs
  client.rs
  template.rs
Cargo.toml
```

## Features
- Use proc macro for `#[agent]` attribute
- `seren test` — test in Daytona
- `seren publish` — upload to marketplace

## How to test
1. Unit tests
2. Integration test with proc macro
3. CLI tests

## References
- Design spec: `docs/plans/00_design_spec.md`