docs: fix README — OPENROUTER_API_KEY is optional for most openrouter-models commands; fix 300 vs 600+ model count

Problems

1. README incorrectly requires API key for all scripts

The README states under the Environment section:

All scripts require an OPENROUTER_API_KEY environment variable.

This is incorrect. openrouter-models explicitly works without a key for most commands — list-models.ts, search-models.ts, compare-models.ts, and resolve-model.ts all call optionalApiKey() and only get-endpoints.ts requires a key. The blanket requirement discourages keyless exploration and is factually wrong.

Fix: Update the README to say key is optional for most commands and only required for endpoint performance data.

2. Model count inconsistency

The README says "600+ AI models" while openrouter-models/SKILL.md and openrouter-typescript-sdk/SKILL.md both say "300+". Pick one and apply it consistently across all skills and the README.

Reviewed by Perry