fix(agent): skill_search tokenizer keeps single CJK runes

cloudAgentSkillSearchTokens dropped any token shorter than 2 runes.
That filter was copied from English stop-word logic (a / I), but a single
Han rune such as the ones meaning hook, gag or twist is already a complete
semantic unit in Chinese. When the whole keyword consisted of such runes,
tokens came back empty and cloudAgentSearchSkills degraded to listing the
full index of enabled skills - the user thinks a search happened but gets
an unsorted dump.

Fix: keep a token when it is exactly one Han rune (unicode.Han); single
ASCII/digit runes and empty strings are still dropped as before.

Test: TestCloudAgentSkillSearchTokensKeepsSingleCJKRune fails on the old
logic (got [反转 钩子], want [反转 钩子 梗 钩]) and passes after the fix.
recall_lessons has an isomorphic copy of the same tokenizer, fixed
separately in #593.

backend/internal/app/cloud_agent_tools.go

@@ -68,7 +68,12 @@ func cloudAgentSkillSearchTokens(keyword string) []string {
 	seen := make(map[string]bool, cloudAgentSkillSearchTokenMax)
 	for _, raw := range strings.FieldsFunc(keyword, cloudAgentSkillTokenSeparator) {
 		token := strings.ToLower(strings.TrimSpace(raw))
-		if utf8.RuneCountInString(token) < 2 || seen[token] {
+		// 单字停用词过滤照拿的是英文逻辑（a / I）；汉字单字「梗」「钩」「戏」本身是完整
+		// 语义的最小单位。一刀切丢后 tokens 为空，检索会退化成「列全部已启用技能索引」。
+		// 这里只放行单个汉字的 token，英文/数字单字与空串仍按停用词丢掉。
+		first, size := utf8.DecodeRuneInString(token)
+		singleHan := size == len(token) && unicode.Is(unicode.Han, first)
+		if (!singleHan && utf8.RuneCountInString(token) < 2) || seen[token] {
 			continue
 		}
 		seen[token] = true