fix(growth): MEDIUM-10 Experience 添加 tool_used 字段

根因: Experience 结构体没有 tool_used 字段，PatternAggregator 从
context 字段提取工具名（语义混淆），导致工具信息不准确。

修复:
- experience_store.rs: Experience 添加 tool_used: Option<String> 字段
  (#[serde(default)] 兼容旧数据)，Experience::new() 初始化为 None
- experience_extractor.rs: persist_experiences() 从 ExperienceCandidate
  的 tools_used[0] 填充 tool_used，同时填充 industry_context
- pattern_aggregator.rs: 改用 tool_used 字段提取工具名，不再误用 context
- store_experience() 将 tool_used 加入 keywords 提升搜索命中率

crates/zclaw-growth/src/experience_store.rs

@@ -48,6 +48,9 @@ pub struct Experience {
     /// Which trigger signal produced this experience.
     #[serde(default)]
     pub source_trigger: Option<String>,
+    /// Primary tool/skill used to resolve this pain point.
+    #[serde(default)]
+    pub tool_used: Option<String>,
 }
 
 impl Experience {
@@ -72,6 +75,7 @@ impl Experience {
             updated_at: now,
             industry_context: None,
             source_trigger: None,
+            tool_used: None,
         }
     }
 
@@ -124,6 +128,9 @@ impl ExperienceStore {
         if let Some(ref industry) = exp.industry_context {
             keywords.push(industry.clone());
         }
+        if let Some(ref tool) = exp.tool_used {
+            keywords.push(tool.clone());
+        }
 
         let entry = MemoryEntry {
             uri,