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/GeminiAgent.ts

@@ -175,7 +175,9 @@ export class GeminiAgent {
           worker,
           tokensUsed,
           null,
-          'Gemini'
+          'Gemini',
+          undefined,
+          model
         );
       } else {
         logger.error('SDK', 'Empty Gemini init response - session may lack context', {
@@ -248,7 +250,8 @@ export class GeminiAgent {
               tokensUsed,
               originalTimestamp,
               'Gemini',
-              lastCwd
+              lastCwd,
+              model
             );
           } else {
             logger.warn('SDK', 'Empty Gemini observation response, skipping processing to preserve message', {
@@ -298,7 +301,8 @@ export class GeminiAgent {
               tokensUsed,
               originalTimestamp,
               'Gemini',
-              lastCwd
+              lastCwd,
+              model
             );
           } else {
             logger.warn('SDK', 'Empty Gemini summary response, skipping processing to preserve message', {