feat: add Gemini API as alternative AI provider

Adds support for Google's Gemini API as an alternative to Claude Agent SDK
for observation extraction. Users can now choose between providers in the
settings UI.

Features:
- New GeminiAgent class using Gemini REST API
- Provider selection in Settings (Claude vs Gemini)
- Gemini API key configuration (via UI or GEMINI_API_KEY env var)
- Model selection: gemini-2.0-flash-exp, gemini-1.5-flash, gemini-1.5-pro
- Graceful fallback to Claude SDK if Gemini selected but no API key
- Seamless transition between providers without worker restart

Settings:
- CLAUDE_MEM_PROVIDER: 'claude' | 'gemini'
- CLAUDE_MEM_GEMINI_API_KEY: API key for Gemini
- CLAUDE_MEM_GEMINI_MODEL: Model selection

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

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

src/shared/SettingsDefaultsManager.ts

@@ -17,6 +17,10 @@ export interface SettingsDefaults {
   CLAUDE_MEM_WORKER_PORT: string;
   CLAUDE_MEM_WORKER_HOST: string;
   CLAUDE_MEM_SKIP_TOOLS: string;
+  // AI Provider Configuration
+  CLAUDE_MEM_PROVIDER: string;  // 'claude' | 'gemini'
+  CLAUDE_MEM_GEMINI_API_KEY: string;
+  CLAUDE_MEM_GEMINI_MODEL: string;  // 'gemini-2.0-flash-exp' | 'gemini-1.5-flash' | 'gemini-1.5-pro'
   // System Configuration
   CLAUDE_MEM_DATA_DIR: string;
   CLAUDE_MEM_LOG_LEVEL: string;
@@ -50,6 +54,10 @@ export class SettingsDefaultsManager {
     CLAUDE_MEM_WORKER_PORT: '37777',
     CLAUDE_MEM_WORKER_HOST: '127.0.0.1',
     CLAUDE_MEM_SKIP_TOOLS: 'ListMcpResourcesTool,SlashCommand,Skill,TodoWrite,AskUserQuestion',
+    // AI Provider Configuration
+    CLAUDE_MEM_PROVIDER: 'claude',  // Default to Claude
+    CLAUDE_MEM_GEMINI_API_KEY: '',  // Empty by default, can be set via UI or env
+    CLAUDE_MEM_GEMINI_MODEL: 'gemini-2.0-flash-exp',  // Default Gemini model
     // System Configuration
     CLAUDE_MEM_DATA_DIR: join(homedir(), '.claude-mem'),
     CLAUDE_MEM_LOG_LEVEL: 'INFO',