feat: multi-turn conversations and Claude fallback for Gemini provider

Major improvements to Gemini provider:

**Shared Conversation History**
- Add ConversationMessage interface for provider-agnostic history
- Both Claude and Gemini agents read/write shared conversationHistory
- Context persists across provider switches via claudeSessionId linkage

**Multi-Turn Gemini API**
- Replace stateless single-query with full conversation context
- queryGeminiMultiTurn() sends entire history for coherent responses
- Maps 'assistant' role to 'model' for Gemini API compatibility

**Automatic Fallback to Claude**
- Detect rate limits (429), server errors (5xx), network failures
- Fall back to Claude SDK when Gemini API fails
- Reset 'processing' messages to 'pending' before fallback

**Mid-Session Provider Switching**
- Track currentProvider on ActiveSession
- Provider changes take effect after current generator finishes
- Avoids race conditions from aborting active generators

Files changed:
- worker-types.ts: Add ConversationMessage, currentProvider tracking
- GeminiAgent.ts: Multi-turn queries, fallback logic
- SDKAgent.ts: Capture messages to shared history
- SessionManager.ts: Initialize new session fields
- SessionRoutes.ts: Provider selection and switching logic
- worker-service.ts: Wire up fallback agent dependency

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/services/worker/SessionManager.ts

@@ -117,7 +117,9 @@ export class SessionManager {
       startTime: Date.now(),
       cumulativeInputTokens: 0,
       cumulativeOutputTokens: 0,
-      pendingProcessingIds: new Set()
+      pendingProcessingIds: new Set(),
+      conversationHistory: [],  // Initialize empty - will be populated by agents
+      currentProvider: null  // Will be set when generator starts
     };
 
     this.sessions.set(sessionDbId, session);