dx: replace 'any' types in cost-extractor.ts with proper interfaces

[George-iam]

The cost extractor utility (src/utils/cost-extractor.ts) uses any types in two places: extractCostFromResult(msg: any) and when iterating over msg.modelUsage as Record<string, any>. Since the function processes Claude SDK response messages, the input shape is known and can be typed with a proper interface. Replacing the any types improves type safety and IDE autocompletion.

Relevant files:

• src/utils/cost-extractor.ts — define an interface for the expected message shape and replace any usages

Acceptance criteria:

• No any types remain in the file
• A new interface (e.g., ClaudeResultMessage) defines the expected input shape with optional fields
• npm run lint passes (tsc --noEmit with strict mode)
• No runtime behavior change