Refactor usage constants into factory functions

[Pratham-ds]

Refactor usage and model constants into factory functions in Agent

• Replaces shared EMPTY_USAGE and DEFAULT_MODEL constants with createEmptyUsage() and createDefaultModel() factory functions so each caller gets a unique object instance, preventing shared-state mutations.
• Agent.reset now throws if called while a run is active; callers must wait for idle before resetting.
• Agent.finishRun clears activeRun before resolving the run promise, so isProcessing is false during resolution callbacks.
• Agent.processEvents iterates over a snapshot of listeners, so subscribe/unsubscribe calls during dispatch take effect only on subsequent events.
• Adds Agent.isProcessing getter that returns true when a run is active.
• Behavioral Change: reset during an active run now throws instead of silently clearing state.

^{Macroscope summarized 2850a37.}