fix: add context window management to prevent runaway API costs

Implements critical safeguards for OpenRouter integration:

- Context truncation: sliding window keeps max 20 messages (configurable)
- Token limits: hard cap at 100k estimated tokens per request
- Cost tracking: logs token usage and estimated cost per API call
- High-usage warnings: alerts when single request exceeds 50k tokens

New settings:
- CLAUDE_MEM_OPENROUTER_MAX_CONTEXT_MESSAGES (default: 20)
- CLAUDE_MEM_OPENROUTER_MAX_TOKENS (default: 100000)

Prevents exponential context growth that caused $300 runaway charges
in initial testing. Context now automatically truncates to most recent
messages within token budget, with detailed logging for monitoring.

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

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

src/shared/SettingsDefaultsManager.ts

@@ -26,6 +26,8 @@ export interface SettingsDefaults {
   CLAUDE_MEM_OPENROUTER_MODEL: string;
   CLAUDE_MEM_OPENROUTER_SITE_URL: string;
   CLAUDE_MEM_OPENROUTER_APP_NAME: string;
+  CLAUDE_MEM_OPENROUTER_MAX_CONTEXT_MESSAGES: string;
+  CLAUDE_MEM_OPENROUTER_MAX_TOKENS: string;
   // System Configuration
   CLAUDE_MEM_DATA_DIR: string;
   CLAUDE_MEM_LOG_LEVEL: string;
@@ -68,6 +70,8 @@ export class SettingsDefaultsManager {
     CLAUDE_MEM_OPENROUTER_MODEL: 'anthropic/claude-3.5-sonnet',  // Default OpenRouter model
     CLAUDE_MEM_OPENROUTER_SITE_URL: '',  // Optional: for OpenRouter analytics
     CLAUDE_MEM_OPENROUTER_APP_NAME: 'claude-mem',  // App name for OpenRouter analytics
+    CLAUDE_MEM_OPENROUTER_MAX_CONTEXT_MESSAGES: '20',  // Max messages in context window
+    CLAUDE_MEM_OPENROUTER_MAX_TOKENS: '100000',  // Max estimated tokens (~100k safety limit)
     // System Configuration
     CLAUDE_MEM_DATA_DIR: join(homedir(), '.claude-mem'),
     CLAUDE_MEM_LOG_LEVEL: 'INFO',