docs(PersonMentionEditor): document client-side fetch exception inline

Per Markus #5616, the leaf-component fetch in the Tiptap suggestion plugin
violates the project-wide rule from frontend/CLAUDE.md ("Data flows from
+page.server.ts via props — never client-side API fetch"). Add an inline
block-comment explaining why this exception is justified (suggestion runs
client-side per keystroke; same auth surface; no server-side reshape
benefit) and points future readers at the open ADR follow-up plus Nora's
PersonSummaryDTO response-shape audit.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/components/PersonMentionEditor.svelte

@@ -123,6 +123,20 @@ onMount(() => {
 				},
 				suggestion: {
 					char: '@',
+					// ─────────────────────────────────────────────────────────────
+					// EXCEPTION to frontend/CLAUDE.md "no client-side API fetch":
+					// Tiptap's suggestion plugin lives entirely on the client and
+					// fires on every keystroke after `@`. Routing each query through
+					// a SvelteKit form action would round-trip through SSR for a
+					// dropdown that needs to feel instantaneous, and a +server.ts
+					// endpoint would only proxy the same call. Auth flows through
+					// the Vite proxy in dev and Caddy in prod (cookie-based), so the
+					// network surface is identical to a server-driven call.
+					// Markus #5616: an ADR will formalise this. Open follow-up:
+					// "ADR: client-side fetch exception for editor suggestion plugins."
+					// Nora #5618 #3 — separate issue tracks the GET /api/persons
+					// response-shape audit (PersonSummaryDTO leaks `notes`).
+					// ─────────────────────────────────────────────────────────────
 					items: async ({ query }: { query: string }) => {
 						if (!query) return [];
 						try {