refactor: remove decisionkind surface

[rlippmann]

What changed

• removed DecisionKind from the runtime and root public API surface
• updated the root API contract fixture and related tests to stop expecting DecisionKind
• rewrote affected tests to use DECISION_* constants instead of the removed enum

Why

• DecisionKind had become redundant with the string-literal Decision union and the DECISION_* public constants
• keeping a single named public decision-kind vocabulary reduces surface area without changing wire behavior
• related to Review public API mutation boundaries #201

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)