[codex] Add message schemas and external clients

[superposition]

Closes #41

Summary

• add leash-schema, a Rust-generated JSON Schema artifact for core Leash wire messages
• add checked-in schemas and compatibility/versioning docs
• add standard-library Python and Node clients that read health/telemetry and call stop against sim HTTP
• wire schema freshness and client examples into CI/smoke coverage

Validation

• cargo fmt --check
• cargo run --features mcp --bin leash-schema -- --check
• bash -n scripts/smoke-all.sh scripts/smoke-http.sh
• node --check examples/clients/node/http-client.mjs
• python3 -m py_compile examples/clients/python/http_client.py
• cargo check --all-features
• scripts/smoke-http.sh
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-targets --all-features
• cargo test --no-default-features --features sim,mcp
• scripts/smoke-all.sh
• cargo package --locked --allow-dirty
• cargo package --locked