feat(chat): make web search built-in (always attach native web_search)

handle_chat_agentic now always attaches Anthropic's native web_search_20250305
tool instead of gating it behind a per-request web_search flag. Web search is a
built-in capability: the model invokes it only when a query needs fresh info
(max_uses:5 caps it), so there is no user-facing toggle. The body's web_search
field is now ignored (back-compat — old UI clients sending it cause no harm).

Pairs with neuron-ui removing the chat-input web search toggle.
Note: .el change only — no elc on the authoring machine; reviewer builds/verifies.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

chat.el

@@ -259,6 +259,17 @@ fn agentic_tools_literal() -> String {
     "]"
 }
 
+// agentic_tools_with_web — the standard tool set, always plus Anthropic's NATIVE
+// server-side web_search tool. Web search is BUILT IN: the model invokes it only when a
+// query needs fresh info (max_uses caps it), so there is no user-facing toggle. The native
+// tool is executed by Anthropic (not by the soul), so it returns real results with citations
+// and needs no local runtime — it sidesteps the soul's lack of executable tools entirely.
+fn agentic_tools_with_web() -> String {
+    let base: String = agentic_tools_literal()
+    let inner: String = str_slice(base, 1, str_len(base) - 1)
+    return "[" + inner + ",{\"type\":\"web_search_20250305\",\"name\":\"web_search\",\"max_uses\":5}]"
+}
+
 fn dispatch_tool(tool_name: String, tool_input: String) -> String {
     if str_eq(tool_name, "read_file") {
         let path: String = json_get(tool_input, "path")
@@ -303,7 +314,7 @@ fn handle_chat_agentic(body: String) -> String {
     let system: String = identity + " You have access to tools: read files, write files, browse the web, search your memory, run commands. Use them when they add genuine value. Be direct.\n\n" + ctx
 
     let api_key: String = agentic_api_key()
-    let tools_json: String = agentic_tools_literal()
+    let tools_json: String = agentic_tools_with_web()
     let safe_msg: String = json_safe(message)
     let safe_sys: String = json_safe(system)
     let messages: String = "[{\"role\":\"user\",\"content\":\"" + safe_msg + "\"}]"