feat(caldav): add slot support for feed items

Adds three LLM-fillable slots to every CalDav feed item:
insight, preparation, and crossSource. Slot prompts are
stored in separate .txt files under src/prompts/ with
few-shot examples to steer the LLM away from restating
event details.

Co-authored-by: Ona <no-reply@ona.com>

packages/aris-source-caldav/src/text.d.ts

@@ -0,0 +1,4 @@
+declare module "*.txt" {
+	const content: string
+	export default content
+}