fix: add cacheLife("max") to build-time cached functions and clarify throw-vs-catch caching rule

- Add `cacheLife("max")` to `getLocalImage` (OG image) and `getStaticRoutes` (sitemap) so entries are held as long as possible during prerender instead of expiring at the default TTL
- Update AGENTS.md to distinguish build-time reads (catch inside the cached function, return a fallback, use a short `cacheLife` on the error path) from request-time reads (may throw from the cached function since they never run during prerender)
- Expand the `getCachedComments` comment to explain why throwing is only safe there: a throw inside `"use cache"` during prerender fails the build even if the caller catches it

app/notes/[slug]/opengraph-image.tsx

@@ -1,6 +1,7 @@
 import fs from "node:fs";
 import path from "node:path";
 
+import { cacheLife } from "next/cache";
 import { notFound } from "next/navigation";
 import { ImageResponse } from "next/og";
 
@@ -31,6 +32,7 @@ const loadGoogleFont = async (font: string, weight: number): Promise<ArrayBuffer
 
 const getLocalImage = async (src: string): Promise<ArrayBuffer | string> => {
   "use cache";
+  cacheLife("max");
 
   // https://stackoverflow.com/questions/5775469/whats-the-valid-way-to-include-an-image-with-no-src/14115340#14115340
   const NO_IMAGE = "data:image/gif;base64,R0lGODlhAQABAAD/ACwAAAAAAQABAAACADs=";