Add cross-language message schemas and client examples

[superposition]

Parent: #12

Goal

Let non-Rust modules and tools speak Leash messages cleanly.

Requirements

• Define canonical schema output for core Leash messages.
• Add external client examples consuming telemetry and invoking capabilities.
• Keep schemas generated from Rust types where possible.
• Document versioning and backward compatibility rules.

Acceptance Criteria

• Schema generation is part of CI.
• External client examples can call health and stop against sim HTTP.
• Compatibility rules are documented.

[superposition]

Merged to main via PR #91: #91

Merge commit: 4838e6ff0a506d0413831422fa3300df5e30f941

Local proof:

• 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

PR CI proof: all gh pr checks 91 jobs passed, including both rust jobs and every feature-matrix job.