feat(examples): add npm run clean to reset generated artifacts

All three example apps (euler-multichain, euler-subgraph, uniswap-portal)
lacked a `clean` script, so a `dev`/`codegen`/`start` run left `.ponder`,
`generated`, and `ponder-env.d.ts` behind with no one-shot reset.

Add an identical `"clean": "rm -rf .ponder generated ponder-env.d.ts"` to
each example's `scripts` block. It removes exactly the regenerable,
already-gitignored artifacts and leaves `git status examples/` clean.
`rm -rf` matches the ponder-example convention and keeps the examples
dependency-free (no rimraf); CI is linux. node_modules and
package-lock.json are intentionally left alone — a fast, non-destructive
reset rather than a deps wipe.

Verified live on euler-subgraph: install -> codegen + dev materialize the
three artifacts -> `npm run clean` removes all three -> git status clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dzhelezov

examples/euler-multichain/package.json

@@ -5,7 +5,8 @@
   "scripts": {
     "dev": "ponder dev",
     "start": "ponder start",
-    "codegen": "ponder codegen"
+    "codegen": "ponder codegen",
+    "clean": "rm -rf .ponder generated ponder-env.d.ts"
   },
   "dependencies": {
     "abitype": "^0.10.2",

examples/euler-subgraph/package.json

@@ -10,7 +10,8 @@
     "serve": "ponder serve",
     "summary": "node scripts/summary.mjs",
     "lint": "eslint .",
-    "typecheck": "tsc"
+    "typecheck": "tsc",
+    "clean": "rm -rf .ponder generated ponder-env.d.ts"
   },
   "dependencies": {
     "abitype": "^0.10.2",

examples/uniswap-portal/package.json

@@ -5,7 +5,8 @@
   "scripts": {
     "dev": "ponder dev",
     "start": "ponder start",
-    "codegen": "ponder codegen"
+    "codegen": "ponder codegen",
+    "clean": "rm -rf .ponder generated ponder-env.d.ts"
   },
   "dependencies": {
     "abitype": "^0.10.2",