fix: resolve a per-chain RPC URL on a fork

[apexearth]

rpcUrlFor() returns LOCAL_PROVIDER_URL for every chain whenever FORK=true, so a fork stack can only ever serve one chain. anvil serves a single chain id, so a Sonic or Base action pointed at a mainnet fork fails in initNetwork() — the provider is built for the action's real chain id while the node reports the fork's:

[otokenOsRebase] Running on sonic (146)
[otokenOsRebase] Error: underlying network changed
  (network={"chainId":146,"name":"unknown"},
   detectedNetwork={"name":"homestead","chainId":1}, code=NETWORK_ERROR)

-  if (process.env.FORK === "true" && process.env.LOCAL_PROVIDER_URL) {
-    url = process.env.LOCAL_PROVIDER_URL;
+  const forkUrl =
+    process.env.FORK === "true"
+      ? process.env[`LOCAL_PROVIDER_URL_${chainId}`] || process.env.LOCAL_PROVIDER_URL
+      : undefined;
+  if (forkUrl) {
+    url = forkUrl;

A per-chain LOCAL_PROVIDER_URL_<chainId> wins when set, falling back to the shared LOCAL_PROVIDER_URL.

Scenario	chain 1	chain 146	chain 8453
Multi-chain fork	anvil:8545	anvil-sonic:8545	anvil-base:8545
Single fork (back-compat)	anvil:8545	anvil:8545	anvil:8545
No fork	MAINNET_PROVIDER_URL	SONIC_PROVIDER_URL	BASE_PROVIDER_URL

Single-chain forks behave exactly as before, and the non-fork path is untouched.

Why now

oplabs/talos#30 runs one anvil per chain (mainnet, Base, Sonic) so actions on all three can be exercised locally against an impersonated relayer. Companion PR in this repo: #2976 (nonce-queue recording on the impersonated branch).

Not verified

No typecheck — the worktree has no node_modules. Resolution logic was checked against all three scenarios above by simulation, not by running the code.