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/hooks/keyword-detector/index.ts

@@ -2,7 +2,7 @@ import type { PluginInput } from "@opencode-ai/plugin"
 import { detectKeywordsWithType, extractPromptText, removeCodeBlocks } from "./detector"
 import { log } from "../../shared"
 import { isSystemDirective } from "../../shared/system-directive"
-import { getMainSessionID } from "../../features/claude-code-session-state"
+import { getMainSessionID, getSessionAgent } from "../../features/claude-code-session-state"
 import type { ContextCollector } from "../../features/context-injector"
 
 export * from "./detector"
@@ -30,7 +30,8 @@ export function createKeywordDetectorHook(ctx: PluginInput, collector?: ContextC
         return
       }
 
-      let detectedKeywords = detectKeywordsWithType(removeCodeBlocks(promptText), input.agent)
+      const currentAgent = getSessionAgent(input.sessionID) ?? input.agent
+      let detectedKeywords = detectKeywordsWithType(removeCodeBlocks(promptText), currentAgent)
 
       if (detectedKeywords.length === 0) {
         return