docs: README adoption upgrade

[slegarraga]

Summary

• Added install size (packagephobia) and min+gzip (bundlephobia) badges to the badge row
• Tightened tagline to a concrete compound form: one normalized error shape, four providers, zero deps
• Added Why not X? section comparing against axios-retry/p-retry, SDK built-in retries, and rolling your own
• Added Integrations section with three real SDK call-site snippets: OpenAI, Anthropic, and Vercel AI SDK, each showing normalizeError + getRetryDelayMs retry branching with accurate SDK shapes
• Moved security-posture link below the Integrations section (problem statement first)
• Expanded Related section to include all four siblings: json-from-llm, tool-schema, llm-sse, llm-messages
• All local CI checks pass: typecheck, lint, 83 tests, format:check, build

Test plan

• CI passes on Node 20 and 22
• Verify badge images render on the GitHub README preview
• Spot-check Integrations code snippets match the actual SDK types