feat: Anthropic SDK with prompt caching support

[rubenmarcus]

Summary

• Replaces raw fetch calls to Anthropic API with the official @anthropic-ai/sdk
• Enables prompt caching via cache_control: { type: "ephemeral" } on system messages
• Cache reads are 90% cheaper than regular input tokens
• Adds system message field to LLMRequest for all providers (Anthropic, OpenAI, OpenRouter)
• Adds cache-aware pricing tiers to cost tracker (write: 1.25x, read: 0.1x)
• Displays cache savings in CLI output and activity.md summaries

Impact

• Up to 90% reduction in input token costs for repeated system prompts
• Actual API usage metrics instead of text-based estimates (when using Anthropic)
• Singleton client pattern for connection pooling

Files changed

• src/llm/api.ts — Refactored to use Anthropic SDK, added system message + cache support
• src/loop/cost-tracker.ts — Added cache pricing, CacheMetrics, recordIterationWithUsage()
• package.json — Added @anthropic-ai/sdk dependency

Test plan

• npm run build passes
• npm run test:run — all 143 tests pass
• Call tryCallLLM() with system message, verify cache metrics in response
• Run ralph-starter run --track-cost and verify cache savings in cost summary

🤖 Generated with Claude Code

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/prompt-caching

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[rubenmarcus]

Merged into staging/pre-conference branch for pre-release testing. Will be included in the upcoming stable release.