fix(keyword-detector): use session state for agent-specific ultrawork templates

Bug: When switching from Prometheus to Sisyphus, the Prometheus ultrawork
template was still injected because:
1. setSessionAgent() only sets on first call, ignoring subsequent updates
2. keyword-detector relied solely on input.agent which could be stale

Fix:
- Use updateSessionAgent() instead of setSessionAgent() in index.ts
- keyword-detector now uses getSessionAgent() as primary source, fallback to input.agent
- Added tests for agent switch scenario

src/index.ts

@@ -51,6 +51,7 @@ import {
   setMainSession,
   getMainSessionID,
   setSessionAgent,
+  updateSessionAgent,
   clearSessionAgent,
 } from "./features/claude-code-session-state";
 import {
@@ -309,7 +310,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
 
     "chat.message": async (input, output) => {
       if (input.agent) {
-        setSessionAgent(input.sessionID, input.agent);
+        updateSessionAgent(input.sessionID, input.agent);
       }
 
       const message = (output as { message: { variant?: string } }).message
@@ -450,7 +451,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
         const agent = info?.agent as string | undefined;
         const role = info?.role as string | undefined;
         if (sessionID && agent && role === "user") {
-          setSessionAgent(sessionID, agent);
+          updateSessionAgent(sessionID, agent);
         }
       }