fix: assistant IA toujours en mode agent, retrait du bouton toggle #187

- Bouton « mode agent » du panneau supprimé : l'assistant est toujours
  agent (toute requête texte part sur /api/ai/bookslm/agent, les images
  restent sur /chat multimodal). Les mutations gardent la confirmation
  two-step. Deep Research et les quick actions `agent: true` basculaient
  déjà le mode en silence : le toggle ne protégeait plus rien.
- /agent résout le provider comme /chat (req.provider brut transmis à
  l'adapter pouvait désigner un fournisseur indisponible et retomber
  silencieusement sur un autre que l'étiquette SSE affichée).
- Schémas des outils mis en cache par (scope, taille du registre) avec
  copies fraîches par appelant (~27 ms de pydantic économisées par
  requête agent/MCP).
- Aide in-app réécrite, i18n FR/EN (retrait de ai.agent_mode_*),
  tests frontend adaptés (ai.test.mjs 100/100) + non-régression pytest
  (provider résolu, cache sûr).

tests/frontend/ai.test.mjs

@@ -159,7 +159,8 @@ async function main() {
       read: async () => (chunks.length ? { done: false, value: chunks.shift() } : { done: true }),
     };
     globalThis.fetch = async (url, opts) => {
-      if (String(url).includes("/api/ai/bookslm/chat")) {
+      // #187: plain-text sends now go to /agent — accept both bookslm endpoints.
+      if (/\/api\/ai\/bookslm\/(chat|agent)/.test(String(url))) {
         capturedBody = JSON.parse(opts.body);
         return { ok: true, status: 200, body: { getReader: () => reader } };
       }
@@ -1171,38 +1172,28 @@ async function main() {
     assert.equal(b._messages[0].content, "Ancienne conversation");
   });
 
-  // ── 14. Agent mode & confirmations (B5) ──
-  await test("agent mode toggle persists and switches the endpoint", async () => {
+  // ── 14. Agent mode & confirmations (B5 / #187) ──
+  await test("#187: no agent toggle — plain requests always target /agent", async () => {
     localStorage.clear();
     const b = new BooksLM();
-    assert.equal(b._agentMode, false, "agent mode off by default");
-    b._toggleAgentMode();
-    assert.equal(b._agentMode, true);
-    assert.equal(localStorage.getItem("obsigate-bookslm-agent"), "true");
-    const b2 = new BooksLM();
-    assert.equal(b2._agentMode, true, "persisted across instances");
+    assert.equal(b._agentMode, undefined, "agent-mode state removed");
+    assert.equal(b._toggleAgentMode, undefined, "toggle method removed");
 
     let url = null;
     globalThis.fetch = async (u) => { url = String(u); return { ok: true, status: 200, json: async () => ({}) }; };
-    b2._abortCtrl = null;
-    await b2._postChat({ message: "x" });
-    assert.ok(url.includes("/api/ai/bookslm/agent"), "agent mode targets /agent");
-    b2._agentMode = false;
-    await b2._postChat({ message: "x" });
-    assert.ok(url.includes("/api/ai/bookslm/chat"), "default targets /chat");
+    b._abortCtrl = null;
+    await b._postChat({ message: "x" });
+    assert.ok(url.includes("/api/ai/bookslm/agent"), "default targets /agent");
+    assert.equal(localStorage.getItem("obsigate-bookslm-agent"), null, "no legacy persistence");
     localStorage.clear();
   });
 
-  await test("header exposes the agent-mode toggle", () => {
+  await test("header no longer exposes the agent-mode toggle", () => {
     const b = new BooksLM();
     const panel = b._render();
     b._panel = panel;
     document.body.appendChild(panel);
-    const btn = panel.querySelector(".bookslm-btn-agent");
-    assert.ok(btn, "agent toggle button present");
-    assert.equal(btn.getAttribute("aria-pressed"), "false");
-    btn.click();
-    assert.equal(btn.getAttribute("aria-pressed"), "true");
+    assert.equal(panel.querySelector(".bookslm-btn-agent"), null, "toggle button removed");
     panel.remove();
     localStorage.clear();
   });
@@ -1236,7 +1227,6 @@ async function main() {
     };
 
     const b = new BooksLM();
-    b._agentMode = true;
     b._panel = b._render();
     document.body.appendChild(b._panel);
     const msg = {
@@ -1418,7 +1408,7 @@ async function main() {
 
     let posted = null;
     globalThis.fetch = async (url, opts) => {
-      if (String(url).includes("/api/ai/bookslm/chat")) posted = JSON.parse(opts.body);
+      if (/\/api\/ai\/bookslm\/(chat|agent)/.test(String(url))) posted = JSON.parse(opts.body);
       return { ok: true, status: 200, body: { getReader: () => ({ read: async () => ({ done: true }) }) } };
     };
     await b._sendMessage();
@@ -1432,10 +1422,9 @@ async function main() {
     localStorage.clear();
   });
 
-  // ── 32. Images force the plain chat endpoint (not the agent) (#81) ──
-  await test("images route to /chat even in agent mode", async () => {
+  // ── 32. Images force the plain chat endpoint (#81 / #187: always-on agent) ──
+  await test("images route to /chat, text to /agent", async () => {
     const b = new BooksLM();
-    b._agentMode = true;
     let url = "";
     globalThis.fetch = async (u) => {
       url = String(u);