fix(hermes-model-switch): clear stale 'context_length' when switching model in config.yaml

When the user switches model via the model picker, hermes_update_model rewrites
model.default and model.provider in config.yaml but historically left model.context_length
untouched. The Hermes kernel (8ac351407, May 2026) now actively clears that field on
model switch because the previous model's context window almost never matches the new
model — leaving the stale value caused 'context too large' errors and silently truncated
output. Mirror the upstream behavior by dropping the context_length line as we walk the
model: block, so Hermes falls back to the new model's default window.

src-tauri/src/commands/hermes.rs

@@ -2278,6 +2278,12 @@ pub async fn hermes_update_model(
                 provider_written = true;
                 continue;
             }
+            // 与 Hermes 内核 8ac351407 保持一致：切模型时清掉旧 context_length，
+            // 否则新模型会沿用上一个模型的 context window（典型表现：context 报错
+            // / 输出被截断）。删除该行即可，Hermes 会按新模型默认窗口生效。
+            if trimmed.starts_with("context_length:") {
+                continue;
+            }
         }
         out.push(line.to_string());
     }