ai: wire arcadia-search backend (search_kb + read_chunk + reindex button)

Adds the agent-facing surface for the new Tantivy lexical search service
(arcadia-search). Sits alongside the existing search_docs (browser
MiniSearch) — agent picks based on tool description.

- admin-tools.ts: new search_kb(query, corpus, limit?, tags?) and
  read_chunk(chunk_id, corpus) tools. KB_BASE_URL honors
  window.__ARCADIA_SEARCH_URL runtime override + VITE_ARCADIA_SEARCH_URL
  build env, defaults to localhost:7800. Token resolved per-call from
  sessionStorage.arcadia_access_token (matching lib-arcadia-client's
  storage convention) with "dev" fallback for unauthenticated dev.
- assistant.tsx: system-prompt section telling the agent when to pick
  search_docs (browser, bundled) vs search_kb (server, dynamic +
  expandable via read_chunk).
- ai.tsx: reindexKB() helper + "reindex kb (docs)" button on the empty
  state, next to the existing block-preview button. Toasts on
  start/success/failure. Wired with data-action="kb-reindex-docs" so
  the agent can also trigger via the command bus.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/routes/ai.tsx

@@ -111,6 +111,48 @@ function ToolResultBlock({ name, result }: { name: string; result: unknown }) {
   return <div className="px-1">{rich}</div>
 }
 
+// Trigger a server-side rebuild of an arcadia-search corpus. Reads the
+// same KB URL + token resolution as the search_kb tool (see admin-tools.ts).
+// Surfaces success/failure via the existing toast provider.
+async function reindexKB(
+  corpus: string,
+  toast: ReturnType<typeof useToast>,
+): Promise<void> {
+  const baseUrl =
+    (typeof window !== "undefined" &&
+      (window as unknown as { __ARCADIA_SEARCH_URL?: string }).__ARCADIA_SEARCH_URL) ||
+    "http://127.0.0.1:7800"
+  const token =
+    (typeof window !== "undefined" &&
+      window.sessionStorage.getItem("arcadia_access_token")) ||
+    "dev"
+  const url = `${baseUrl}/index/${encodeURIComponent(corpus)}/build`
+  toast.show?.({
+    title: "Reindexing…",
+    description: `Rebuilding corpus '${corpus}'.`,
+  })
+  try {
+    const res = await fetch(url, {
+      method: "POST",
+      headers: { Authorization: `Bearer ${token}` },
+    })
+    if (!res.ok) {
+      throw new Error(`HTTP ${res.status}: ${await res.text()}`)
+    }
+    const out = (await res.json()) as { chunk_count: number; built_at: string }
+    toast.show?.({
+      title: "Reindex complete",
+      description: `${out.chunk_count} chunks indexed for '${corpus}'.`,
+    })
+  } catch (err) {
+    toast.show?.({
+      title: "Reindex failed",
+      description: err instanceof Error ? err.message : String(err),
+      tone: "error",
+    })
+  }
+}
+
 // Synthetic assistant message that exercises every typed rich-output block.
 // Wired to the "preview rich-output blocks" button in the empty state — used
 // to eyeball renderer + theme without driving a live model. Safe to delete
@@ -1179,7 +1221,7 @@ function ChatSurface({
             Issue an instruction. Read tools run automatically. Writes pause for
             confirmation. Tab&nbsp;⇥ for command palette.
           </p>
-          <div className="console-empty-line pointer-events-auto">
+          <div className="console-empty-line pointer-events-auto flex flex-wrap gap-2">
             <button
               type="button"
               onClick={() =>
@@ -1191,6 +1233,14 @@ function ChatSurface({
             >
               › preview rich-output blocks
             </button>
+            <button
+              type="button"
+              onClick={() => void reindexKB("docs", toast)}
+              className="console-mono inline-flex items-center gap-1.5 rounded-md border border-[var(--console-rule-soft)] bg-transparent px-2.5 py-1 text-[10.5px] uppercase tracking-[0.18em] text-[var(--console-muted)] transition-colors hover:border-[var(--console-amber)] hover:text-[var(--console-amber)]"
+              data-action="kb-reindex-docs"
+            >
+              › reindex kb (docs)
+            </button>
           </div>
         </div>
       </div>

app/routes/assistant.tsx

@@ -113,6 +113,7 @@ function buildAdminPreface(activeAgent: Agent | undefined, uiControl: boolean):
   const ctx = formatAdminContextForPrompt()
   const parts = [
     "You are the operator's assistant inside Arcadia Admin. Be precise and direct. You have native function tools attached to this conversation — call them whenever the user asks about live platform state (counts, statuses, listings, lookups). Never invent tenant slugs, user counts, or statuses; if you need data, call a tool.",
+    "Two retrieval surfaces exist for documentation/knowledge: `search_docs` (browser-side, BM25 over the bundled arcadia docs — fast, always available, small corpus) and `search_kb` (server-side, BM25 over arcadia-search — same docs as `corpus=docs` for parity, plus larger and additional corpora as the operator adds them). For questions about the bundled arcadia docs either is fine; prefer `search_kb` when you want richer hits or when the user is asking about content that wouldn't be in the bundled docs (uploaded files, tenant-specific knowledge). When `search_kb` returns a chunk_id you want to expand, call `read_chunk(chunk_id, corpus)`.",
     RICH_OUTPUT_PREFACE,
     ARCADIA_KNOWLEDGE,
     persona,