docs: update READMEs to reference pnpm instead of yarn

[pyramation]

docs: update READMEs to reference pnpm instead of yarn

Summary

Follow-up to #595 (yarn → pnpm migration). Updates all README files that still referenced yarn commands to use pnpm equivalents.

6 files changed:

• README.md — yarn / yarn build → pnpm install / pnpm run build
• packages/cosmos-kit/README.md — same as above
• packages/react/README.md — yarn add → pnpm add
• packages/react-lite/README.md — yarn add → pnpm add
• examples/cosmos-kit-nextjs-pages-router-example/README.md — simplified to pnpm dev
• examples/cosmos-kit-nextjs-app-router-example/README.md — simplified to pnpm dev

Review & Testing Checklist for Human

• Verify no other files in the repo still reference yarn (e.g. CONTRIBUTING.md, wiki, or inline code comments) — a quick grep -r "yarn" --include="*.md" should confirm

Notes

Link to Devin run: https://app.devin.ai/sessions/470239005b9b4c909a5eaf94cd127fce
Requested by: @pyramation

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