Add query_hand_ephemeral one-shot hand query

[marcosvillegasmadrigal]

Introduce a new KernelHandle method query_hand_ephemeral for synchronous one-shot hand queries that return an LLM response wrapped as external untrusted content (hand://...). Implement the method on OpenFangKernel; it clones the agent manifest, applies a transient max_tokens override, disables tools, calls the resolved LLM driver with a wall-clock timeout, and guarantees no session or persistence side effects. Make default_driver test-swappable by changing it to RwLock<Arc> and add a #[cfg(test)] set_test_default_driver helper. Update resolve_driver fallback to read the locked default driver. Add extensive tests and test drivers (RecordingDriver, SleepDriver) to verify wrapping, timeout, max_tokens behavior, and zero-persistence guarantees. Add default trait impl in runtime that returns not supported for backwards compatibility.

Summary

Changes

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes
• Live integration tested (if applicable)

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries