feat(recall): cap injected memory context (#71)

- Add `recall.maxCharsPerMemory` and `recall.maxTotalRecallChars` with defaults of `0`, which do not alter existing behavior. Users can opt in by setting positive values to cap injected memory context.
- Apply the budget after L1 search and before `<relevant-memories>` injection, preserving score order while truncating oversized entries and dropping overflow.
- Document the new guards in README, README_CN, and `openclaw.plugin.json`.

openclaw.plugin.json

@@ -69,6 +69,8 @@
         "properties": {
           "enabled": { "type": "boolean", "default": true, "description": "是否启用自动召回" },
           "maxResults": { "type": "number", "default": 5, "description": "召回最大结果数" },
+          "maxCharsPerMemory": { "type": "number", "default": 0, "description": "单条 L1 记忆注入的最大字符数；填 0 表示不限制" },
+          "maxTotalRecallChars": { "type": "number", "default": 0, "description": "本轮 auto-recall 注入的 L1 记忆总字符预算；填 0 表示不限制" },
           "scoreThreshold": { "type": "number", "default": 0.3, "description": "最低分数阈值" },
           "strategy": { "type": "string", "enum": ["embedding", "keyword", "hybrid"], "default": "hybrid", "description": "搜索策略：keyword(关键词)、embedding(向量)、hybrid(混合RRF融合，推荐)" },
           "timeoutMs": { "type": "number", "default": 5000, "description": "召回整体超时（毫秒），超时后跳过记忆注入并打印警告日志" }