Added a baseline repository and distribution license policy

[konojunya]

Background

As Render Toolchain Task 3, we will define responsibility boundaries, license boundaries, and set up a minimal CI before implementing the engine. The required checks will be configured in the ruleset after verifying the actual successful workflow job names.

Implementation Details

• Document the planned boundaries for the pure Rust engine, formatter, and WASM adapter in the README
• Document in AGENTS.md that compiler/theme dependencies, as well as host I/O, auth, and billing, are outside the scope of responsibilities
• Maintain the existing Apache-2.0 LICENSE
• Add a ledger to track dependency licenses before native/WASM distribution
• Add a minimal GitHub Actions workflow containing the Engine repository job

Verification Steps

• git diff --check "$(git hash-object -t tree /dev/null)" HEAD
• Parse the workflow YAML
• Verify the existence and non-emptiness of README, AGENTS, LICENSE, and THIRD_PARTY_LICENSES
• Verify via SHA-256 that LICENSE contains the exact full text of Apache-2.0, identical to existing Stack public repositories

Review Points / Concerns

This PR only targets the repository foundation and does not anticipate crate structures or public APIs. When actual code is introduced, Rust/WASM-specific quality gates will be added in the same change.