fix(captain): add activity message when resolve tool is used

- Add resolve tool to Captain assistant agent_tools
- Add Current.tool_name to track active tool execution
- Set tool_name in resolve tool before resolving conversation
- Add tool_resolved i18n key for tool-based resolution
- Refactor ActivityMessageHandler for better readability

Fixes issue where conversations resolved via the resolve tool would
silently change status with no timeline/audit entry.

lib/current.rb

@@ -4,6 +4,7 @@ module Current
   thread_mattr_accessor :account_user
   thread_mattr_accessor :executed_by
   thread_mattr_accessor :contact
+  thread_mattr_accessor :tool_name
 
   def self.reset
     Current.user = nil
@@ -11,5 +12,6 @@ module Current
     Current.account_user = nil
     Current.executed_by = nil
     Current.contact = nil
+    Current.tool_name = nil
   end
 end