chore(codegen): restore minimal wagmi.config.ts (hardhat plugin)

Reverts wagmi.config.ts to the pre-existing simpler shape (hardhat
plugin auto-discovers artifacts) and drops our hand-rolled readFileSync
bootstrap. The plugin emits ABIs for every compiled contract; unused
ones are tree-shaken out of the production bundle, and the file itself
is gitignored so the extra bytes never hit the repo.

The plugin runs `npx hardhat compile` internally, which loads
hardhat.config.ts at import time and throws when ALCHEMY_API_KEY is
missing — env vars from our wrapper script don't propagate into the
plugin's subprocess. Fix: prewagmi:gen now bootstraps a placeholder
.env inside the submodule (gitignored there via *.env), instead of
running our own `hardhat compile` with inline env vars.

- wagmi.config.ts: hardhat({ project: 'p2pix-smart-contracts' }), no
  explicit contracts/loadAbi
- package.json: contracts:compile -> contracts:setup
  (bun install + create .env if absent); the wagmi hardhat plugin
  handles the actual compile

package.json

@@ -13,8 +13,8 @@
     "lint:fix": "eslint . --fix",
     "format": "prettier --write \"src/**/*.{ts,vue,json}\"",
     "format:check": "prettier --check \"src/**/*.{ts,vue,json}\"",
-    "contracts:compile": "pushd p2pix-smart-contracts && bun install && ALCHEMY_API_KEY=placeholder MNEMONIC='test test test test test test test test test test test junk' bunx hardhat compile && popd",
-    "prewagmi:gen": "bun run contracts:compile",
+    "contracts:setup": "pushd p2pix-smart-contracts && bun install && ([ -f .env ] || echo 'ALCHEMY_API_KEY=placeholder' > .env) && popd",
+    "prewagmi:gen": "bun run contracts:setup",
     "wagmi:gen": "bun ./node_modules/@wagmi/cli/dist/esm/cli.js generate"
   },
   "dependencies": {