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/SDKAgent.ts

@@ -270,7 +270,8 @@ export class SDKAgent {
             discoveryTokens,
             originalTimestamp,
             'SDK',
-            cwdTracker.lastCwd
+            cwdTracker.lastCwd,
+            modelId
           );
         }