Add hooks, @file mentions, session search/fork/export, follow-scroll

- Hooks ([[hooks]]): pre_tool, post_tool, user_prompt, stop; sandboxed, exit 2 blocks
- @file mentions with autocomplete; mentioned files/folders are attached to the prompt
- Session full-text search in the sidebar, fork into a copy, export as Markdown/JSON
- Chat follows the newest message until the user scrolls up; jump-to-latest button
- SVG activity spark and thinking icon instead of Unicode/emoji glyphs
- File search ranks name matches first and returns relative paths

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/components/Sidebar.tsx

@@ -1,6 +1,6 @@
 import { cycleThemeMode, getThemeMode, onThemeChange } from "../theme";
-import { useMemo, useSyncExternalStore } from "react";
-import { post } from "../api";
+import { useEffect, useMemo, useState, useSyncExternalStore } from "react";
+import { get, post } from "../api";
 import { fmtRelative, useT, type Key } from "../i18n";
 import { navigate } from "../router";
 import { useStore } from "../store";
@@ -27,6 +27,26 @@ export function Sidebar({ open, activeId, section }: { open: boolean; activeId?:
   const permissions = useStore((s) => s.permissions);
   const questions = useStore((s) => s.questions);
   const me = useStore((s) => s.me);
+  // Full-text search over titles and messages.
+  const [query, setQuery] = useState("");
+  const [hits, setHits] = useState<{ id: string; title: string; cwd: string; agent: string; updated_at: number; snippet: string }[]>();
+  useEffect(() => {
+    const q = query.trim();
+    if (q.length < 2) {
+      setHits(undefined);
+      return;
+    }
+    let alive = true;
+    const timer = setTimeout(() => {
+      get<typeof hits>(`/sessions/search?q=${encodeURIComponent(q)}`)
+        .then((h) => alive && setHits(h ?? []))
+        .catch(() => alive && setHits([]));
+    }, 250);
+    return () => {
+      alive = false;
+      clearTimeout(timer);
+    };
+  }, [query]);
 
   const attention = useMemo(() => {
     const ids = new Set<string>();
@@ -60,9 +80,36 @@ export function Sidebar({ open, activeId, section }: { open: boolean; activeId?:
           + {t("sidebar.newSession")}
         </button>
       </div>
+      <div className="session-search">
+        <input
+          type="search"
+          value={query}
+          placeholder={t("sidebar.search")}
+          aria-label={t("sidebar.search")}
+          onChange={(e) => setQuery(e.target.value)}
+          onKeyDown={(e) => e.key === "Escape" && setQuery("")}
+        />
+      </div>
       <nav className="session-list">
-        {sessions.length === 0 && <div className="empty">{t("sidebar.noSessions")}</div>}
-        {groups.map((g) => (
+        {hits && (
+          <>
+            <div className="session-group">{t("sidebar.searchResults", { n: hits.length })}</div>
+            {hits.length === 0 && <div className="empty">{t("sidebar.noResults")}</div>}
+            {hits.map((h) => (
+              <button key={h.id} className={`session-item ${h.id === activeId ? "active" : ""}`} onClick={() => navigate(`/s/${h.id}`)}>
+                <div className="title">
+                  <span>{h.title || t("session.untitled")}</span>
+                </div>
+                {h.snippet && <div className="sub snippet">{h.snippet}</div>}
+                <div className="sub">
+                  {basename(h.cwd)} · {h.agent} · {fmtRelative(h.updated_at)}
+                </div>
+              </button>
+            ))}
+          </>
+        )}
+        {!hits && sessions.length === 0 && <div className="empty">{t("sidebar.noSessions")}</div>}
+        {!hits && groups.map((g) => (
           <div key={g.key}>
             <div className="session-group">{t(g.key)}</div>
             {g.items.map((s) => (