chore(vendor): import pi 0.85.1

Imported as a linear vendor commit so forge rebases do not replay the upstream root tree.

git-subtree-dir: third_party/pi

git-subtree-split: d981de1229ef899957bbe968bc8dcda02a21f477

third_party/pi/packages/coding-agent/src/experimental/plugin.ts

@@ -0,0 +1,16 @@
+export {
+	type AgentCompactionRequest,
+	AgentController,
+	type AgentNavigationRequest,
+	type AgentOperationError,
+	type AgentOperationResponse,
+	type AgentPromptRequest,
+	type AgentQueueResponse,
+} from "./services/agent-controller.ts";
+export { type PresentationSelectItem, PresentationUI } from "./services/presentation-ui.ts";
+export {
+	type SlashCommandCompletion,
+	type SlashCommandContribution,
+	type SlashCommandRunResult,
+	SlashCommands,
+} from "./services/slash-commands.ts";