feat(config): expose context_window in example config and web UI

Add context_window to config.example.json, the web configuration page
(form model, input field, save handler), and i18n strings (en/zh).
The field is optional — leaving it empty falls back to the 4x max_tokens
heuristic.

config/config.example.json

@@ -5,6 +5,7 @@
       "restrict_to_workspace": true,
       "model_name": "gpt-5.4",
       "max_tokens": 8192,
+      "context_window": 131072,
       "temperature": 0.7,
       "max_tool_iterations": 20,
       "summarize_message_threshold": 20,

web/frontend/src/components/config/config-page.tsx

@@ -144,6 +144,9 @@ export function ConfigPage() {
         const maxTokens = parseIntField(form.maxTokens, "Max tokens", {
           min: 1,
         })
+        const contextWindow = form.contextWindow.trim()
+          ? parseIntField(form.contextWindow, "Context window", { min: 1 })
+          : undefined
         const maxToolIterations = parseIntField(
           form.maxToolIterations,
           "Max tool iterations",
@@ -171,6 +174,7 @@ export function ConfigPage() {
               workspace,
               restrict_to_workspace: form.restrictToWorkspace,
               max_tokens: maxTokens,
+              context_window: contextWindow,
               max_tool_iterations: maxToolIterations,
               summarize_message_threshold: summarizeMessageThreshold,
               summarize_token_percent: summarizeTokenPercent,

web/frontend/src/components/config/config-sections.tsx

@@ -114,6 +114,20 @@ export function AgentDefaultsSection({
         />
       </Field>
 
+      <Field
+        label={t("pages.config.context_window")}
+        hint={t("pages.config.context_window_hint")}
+        layout="setting-row"
+      >
+        <Input
+          type="number"
+          min={1}
+          value={form.contextWindow}
+          onChange={(e) => onFieldChange("contextWindow", e.target.value)}
+          placeholder="131072"
+        />
+      </Field>
+
       <Field
         label={t("pages.config.max_tool_iterations")}
         hint={t("pages.config.max_tool_iterations_hint")}

web/frontend/src/components/config/form-model.ts

@@ -5,6 +5,7 @@ export interface CoreConfigForm {
   restrictToWorkspace: boolean
   allowRemote: boolean
   maxTokens: string
+  contextWindow: string
   maxToolIterations: string
   summarizeMessageThreshold: string
   summarizeTokenPercent: string
@@ -57,6 +58,7 @@ export const EMPTY_FORM: CoreConfigForm = {
   restrictToWorkspace: true,
   allowRemote: true,
   maxTokens: "32768",
+  contextWindow: "",
   maxToolIterations: "50",
   summarizeMessageThreshold: "20",
   summarizeTokenPercent: "75",
@@ -119,6 +121,7 @@ export function buildFormFromConfig(config: unknown): CoreConfigForm {
         ? EMPTY_FORM.allowRemote
         : asBool(exec.allow_remote),
     maxTokens: asNumberString(defaults.max_tokens, EMPTY_FORM.maxTokens),
+    contextWindow: asNumberString(defaults.context_window, EMPTY_FORM.contextWindow),
     maxToolIterations: asNumberString(
       defaults.max_tool_iterations,
       EMPTY_FORM.maxToolIterations,

web/frontend/src/i18n/locales/en.json

@@ -396,6 +396,8 @@
       "allow_remote_hint": "When enabled, shell commands can also run for remote sessions or non-local contexts. When disabled, shell execution stays limited to local safe contexts.",
       "max_tokens": "Max Tokens",
       "max_tokens_hint": "Upper token limit per model response.",
+      "context_window": "Context Window",
+      "context_window_hint": "Model input context capacity in tokens. Leave empty to auto-detect (default: 4x max tokens).",
       "max_tool_iterations": "Max Tool Iterations",
       "max_tool_iterations_hint": "Maximum tool-call loops in a single task.",
       "summarize_threshold": "Summarize Message Threshold",

web/frontend/src/i18n/locales/zh.json

@@ -396,6 +396,8 @@
       "allow_remote_hint": "开启后，来自远程会话或非本地上下文的请求也可以执行 shell 命令；关闭后，仅允许本地安全上下文执行。",
       "max_tokens": "最大 Token 数",
       "max_tokens_hint": "单次模型响应允许的最大 Token 数。",
+      "context_window": "上下文窗口",
+      "context_window_hint": "模型输入上下文容量（Token 数）。留空则自动推算（默认为最大 Token 数的 4 倍）。",
       "max_tool_iterations": "最大工具迭代次数",
       "max_tool_iterations_hint": "单个任务中允许的工具调用循环上限。",
       "summarize_threshold": "触发摘要的消息阈值",