修复: system 锚点避让悬空 tool 调用头防 400

- df-ai 新增 ContextManager::push_system_anchor 统一出口:末条为悬空 ToolCallHead 时插其前,否则尾部追加(不位移前缀)
- 5 处 system 尾部 push 全量收口(压缩成功/兜底提交、警示单点、主题标记、续轮 nudge),从源头杜绝后台压缩摘要/警示楔入工具三元组致上游 400 insufficient tool messages
- 单测 push_system_anchor 两态锁定;cargo test -p df-ai --lib context 155 passed / cargo check -p devflow EXIT 0
- 看板归档:全量完成态条目与段落迁 docs/07-项目管理/todo归档/2026-10-10.md(零措辞改动机械搬运,未完成项110条零丢失/完成项37条全入归档/行数守恒)

src-tauri/src/commands/ai/agentic/mod.rs

@@ -1801,7 +1801,7 @@ async fn prepare_round_inputs(
             let mut session = session_arc.lock().await;
             if session.per_conv.contains_key(conv_id) {
                 let conv = session.conv(conv_id);
-                conv.messages.push(ChatMessage::system(&marker_text));
+                conv.messages.push_system_anchor(ChatMessage::system(&marker_text));
                 tracing::info!(
                     conv_id = %conv_id,
                     iteration,
@@ -2396,7 +2396,7 @@ async fn insert_goal_verify_nudge(
         let conv = session.conv(conv_id);
         // 尾部追加而非 insert_at(0):nudge 紧邻本轮内容语义不变,且不位移既有前缀字节
         // (insert_at(0) 会把整条历史后移,自动前缀缓存/Anthropic 显式缓存全失效)。
-        conv.messages.push(ChatMessage::system(&nudge_text));
+        conv.messages.push_system_anchor(ChatMessage::system(&nudge_text));
         tracing::info!(
             conv_id = %conv_id, iteration, verify_rounds,
             "[GOAL-VERIFY] 目标未验证,注入续轮 nudge(强制续轮)"