Config cascade

Agent configuration cascades from the widest scope to the narrowest:

harness default  →  class (@foom.config)  →  method (@foom.config)  →  per-call (.with())

Each option kind merges by a fixed rule:

• Override (most fields — model, thinking, retries, …): the nearest scope wins.
• Caps (maxBudgetUsd, maxOutputTokens, maxCallDepth, maxTurnDuration): tighten-only — a narrower scope can lower a cap but never raise it.
• System prompt: composes — { append } accumulates onto wider scopes, { replace } discards them and becomes the new base.

@foom.config({ model: "openrouter/deepseek/deepseek-v4-flash", maxBudgetUsd: 1.0 })
export default class extends Program(Input) {
  async main() {
    // Inherits the class model; tightens the budget for this one call.
    await this.agent.with({ maxBudgetUsd: 0.2, thinking: "high" }).prose`…`;
  }

  @foom.config({ thinking: "low" }) // applies whenever this method runs a turn
  @foom.expose
  async cheapStep() { /* … */ }
}

The merge is associative and total, so folding the chain in any grouping yields the same effective config. An unenforceable cap is rejected at engine setup through the typed error channel (FoomtimeConfigError).