feat: split dirigent_tools into individual tools + human @mention override

Feature 1: Split dirigent_tools
- Replace monolithic dirigent_tools (9 actions) with 9 individual tools
- Discord: dirigent_channel_create, dirigent_channel_update, dirigent_member_list
- Policy: dirigent_policy_get, dirigent_policy_set, dirigent_policy_delete
- Turn: dirigent_turn_status, dirigent_turn_advance, dirigent_turn_reset
- Extract shared executeDiscordAction() helper

Feature 2: Human @mention override
- When humanList user @mentions agents, temporarily override turn order
- Only mentioned agents cycle, ordered by their turn order position
- Original order restores when cycle returns to first agent or all NO_REPLY
- New: setMentionOverride(), hasMentionOverride(), extractMentionedUserIds()
- New: buildUserIdToAccountIdMap() for reverse userId→accountId resolution

Bump version to 0.3.0

NEW_FEAT.md

@@ -1,4 +1,4 @@
 # New Features
 
-1. 拆分 dirigent-tools：不再用一个主工具管理多个小工具。
-2. 人类@规则：当来自 humanList 的用户消息包含 <@USER_ID> 时，按被 @ 的 agent 顺序临时覆盖 speaking order 循环；回到首个 agent 后恢复原顺序。
+1. 拆分 dirigent-tools：不再用一个主工具管理多个小工具。 ✅
+2. 人类@规则：当来自 humanList 的用户消息包含 <@USER_ID> 时，按被 @ 的 agent 顺序临时覆盖 speaking order 循环；回到首个 agent 后恢复原顺序。 ✅