Integrate Seren API-key flow

[taariq]

Summary

• add the SerenDB project-based target flow (CLI resolver, remote JobSpec fields, worker/lambda sync) so --api-key users can select projects interactively and reuse saved targets.
• include the new target state persistence, picker, and remote job submission logic
• cover the changes with updated tests (remote integration, target-state mutex, new CLI doctests) and state management docs.

Testing

• cargo fmt
• cargo clippy --all-targets -- -D warnings
• cargo test