chore: consolidate example app gitignores into root

[danielrx-echo]

Summary

• Removes per-app `.gitignore` files from all four example app sources (`nextjs-evm`, `nextjs-svm`, `react-evm`, `react-svm`)
• Merges all patterns into the root `.gitignore` so the monorepo root covers the working tree
• Adds `.gitignore` generation to `generate-example.mjs`: the script now writes a self-contained `.gitignore` into each `dist//` output directory so `.env` is excluded when `push-example.sh` runs `git add -A` in the standalone example repos
• Fixes `.env*` handling in the root gitignore so `.env.example` files remain tracked (`!.env.example` negation)
• `contracts/.gitignore` is unchanged (Foundry-specific: `cache/`, `broadcast/`, `lcov.info`)

Test plan

• Verify `.env.example` files in each example app are still tracked: `git check-ignore -v examples/framework/**/.env.example`
• Verify `.env` and `.env.local` are ignored in example app dirs
• Run `pnpm examples:generate` and confirm `dist//.gitignore` is written with correct content
• Confirm `dist//.env` would be excluded by that `.gitignore` before push