Add canonical prompt cost gate tutorial

[faraa2m]

Summary

• add the canonical tutorial: Add an LLM Prompt Cost Gate to GitHub Actions in 10 Minutes
• link it from the root README, adoption playbook, and llms.txt
• use commerce-api-starter as the concrete demo repo and current Action syntax

Verification

• npm run lint
• npm test
• npm run build
• node packages/cli/dist/index.js ../commerce-api-starter/prompts/product-description.md --model claude-sonnet-4-6,gpt-4o --format markdown,json

No changeset: docs and discoverability links only; no package API or runtime behavior changed.

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Project	Deployment	Actions	Updated (UTC)
tokenometer	Ready	Preview, Comment	May 25, 2026 4:59am