fix: wire generated_by_model into observation write path

The generated_by_model column was added to the observations table in the
Phase 0 governance schema migration but never wired into the INSERT
statements. All 3,878+ observations in production have this field NULL.

This fix threads the model ID from each agent (SDKAgent, GeminiAgent,
OpenRouterAgent) through processAgentResponse() into storeObservation(),
storeObservations(), and storeObservationsAndMarkComplete().

Unblocks Thompson Sampling RFC (#1571) which needs {obs_type}:{model}
as the bandit arm key.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/services/worker/agents/ResponseProcessor.ts

@@ -54,7 +54,8 @@ export async function processAgentResponse(
   discoveryTokens: number,
   originalTimestamp: number | null,
   agentName: string,
-  projectRoot?: string
+  projectRoot?: string,
+  modelId?: string
 ): Promise<void> {
   // Track generator activity for stale detection (Issue #1099)
   session.lastGeneratorActivity = Date.now();
@@ -102,7 +103,8 @@ export async function processAgentResponse(
     summaryForStore,
     session.lastPromptNumber,
     discoveryTokens,
-    originalTimestamp ?? undefined
+    originalTimestamp ?? undefined,
+    modelId
   );
 
   // Log storage result with IDs for end-to-end traceability