fix: intermediate text as standalone messages, preserve progress

Add 'intermediate' phase for assistant text between tool calls.
These are sent as separate Discord messages without finalizing
or disrupting the active progress message.

src/agent-runner.ts

@@ -41,7 +41,7 @@ export interface AgentInput {
 export interface AgentOutput {
   status: 'success' | 'error';
   result: string | null;
-  phase?: 'progress' | 'final' | 'tool-activity';
+  phase?: 'progress' | 'final' | 'tool-activity' | 'intermediate';
   newSessionId?: string;
   error?: string;
 }