feat: define cloud extension contracts

[loonghao]

Summary

• add optional worker identity metadata to worker registration and persisted worker info
• document stable and experimental extension contracts for storage, logs/artifacts, worker identity, and scheduler independence
• link the extension contract note from the architecture docs and README

Validation

• cargo fmt --all
• $env:PYO3_PYTHON='C:\Users\hallong\AppData\Local\Programs\Python\Python312\python.exe'; cargo test --workspace
• $env:PYO3_PYTHON='C:\Users\hallong\AppData\Local\Programs\Python\Python312\python.exe'; cargo clippy --workspace --all-targets -- -D warnings
• npm ci
• npm run build
• git diff --check

Closes #12