fix: address review feedback for #769

- Apply PRIVATE_KEY env-var fallback to UpgradeOptimizer.sol (missed in first pass)
- Add comment on zero-sentinel silent-fallback behaviour in all four scripts
- Remove spurious view modifier from BaseDeploy.run() (violated by vm.readFile)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

onchain/script/BaseDeploy.sol

@@ -3,8 +3,9 @@ pragma solidity ^0.8.19;
 import "forge-std/Script.sol";
 
 contract BaseDeploy is Script {
-    function run() public view {
+    function run() public {
         // Base data
+        // PRIVATE_KEY=0 / empty silently falls back to .secret (0 is an invalid secp256k1 key).
         uint256 privateKey = vm.envOr("PRIVATE_KEY", uint256(0));
         if (privateKey == 0) {
             string memory seedPhrase = vm.readFile(".secret");