fix(rpc): point fork test to live Alchemy endpoint, note shared demo key in README

The public.blastapi.io fork RPC returns a well-formed JSON-RPC error
("Blast API is no longer available"), which starknet-devnet fails to
parse and reports as a broken response instead of a clear upstream
error. Switch to the working Alchemy endpoint used by
base-challenge-template.

Also add a short note pointing learners to get their own free Alchemy
key for sustained use, since the shared demo key in .env.example has
one rate limit across everyone using it. Also note that yarn install
never updates an existing packages/snfoundry/.env, so a .env kept
from an old clone can still point at the now-decommissioned
public.blastapi.io.

Author: tu11aa

README.md

@@ -293,6 +293,10 @@ vendor = await deployContract({
 
 > Find the `packages/snfoundry/.env` file and fill the env variables related to Sepolia testnet with your own wallet account address and private key.
 
+> 💡 The RPC URL shipped in `.env.example` uses a shared demo Alchemy key. That's fine for a quick first run, but for anything sustained - a workshop, a long session, or a mainnet fork - grab your own free key from [Alchemy](https://www.alchemy.com/). The shared key has one rate limit across everyone using it, and when it throttles you won't see a "rate limited" error - you'll get an opaque parse or fork error and may think your own setup is broken.
+>
+> If you already have a `packages/snfoundry/.env` from an earlier clone, compare it against `.env.example` - `yarn install` never touches an existing `.env`, so a stale one may still point at the now-decommissioned `public.blastapi.io`. That failure won't say the endpoint is dead either; devnet just reports a raw parse error that looks like devnet itself is broken.
+
 ⛽️ You will need to get some `STRK` Sepolia tokens to deploy your contract to Sepolia testnet.
 
 🚀 Run `yarn deploy --network [network]` to deploy your smart contract to a public network (mainnet or sepolia).

packages/snfoundry/contracts/Scarb.toml

@@ -31,7 +31,7 @@ sort-module-level-items = true
 
 [[tool.snforge.fork]]
 name = "SEPOLIA_LATEST"
-url = "https://starknet-sepolia.public.blastapi.io/rpc/v0_9"
+url = "https://starknet-sepolia.g.alchemy.com/starknet/version/rpc/v0_10/_hKu4IgnPgrF8O82GLuYU"
 block_id.tag = "latest"
 
 [tool]