feat(ai): support message-anchored tool loading (#6474)

This adds cache-friendly dynamic tool loading anchored to tool results. Purely additive active-tool changes are recorded with `addedToolNames`, allowing supported Anthropic and OpenAI Responses models to load tool definitions at the point they become available instead of placing them in the cached prompt prefix.

It retains safe fallback behavior for unsupported models and non-additive changes but it will wipe caches.

packages/agent/src/agent-loop.ts

@@ -734,6 +734,7 @@ async function finalizeExecutedToolCall(
 			);
 			if (afterResult) {
 				result = {
+					...result,
 					content: afterResult.content ?? result.content,
 					details: afterResult.details ?? result.details,
 					terminate: afterResult.terminate ?? result.terminate,
@@ -779,6 +780,7 @@ function createToolResultMessage(finalized: FinalizedToolCallOutcome): ToolResul
 		// so the null never enters session history or provider payloads.
 		content: finalized.result.content ?? [],
 		details: finalized.result.details,
+		...(finalized.result.addedToolNames?.length ? { addedToolNames: finalized.result.addedToolNames } : {}),
 		isError: finalized.isError,
 		timestamp: Date.now(),
 	};