configs: size each M-Chain policy to the validator set that must satisfy it

mainnet's mchain.json asked for 7-of-10 while mainnet genesis declares
five validators, and localnet asked for 3-of-5 with three. Nothing read
those fields before, so the mismatch was inert; now that the policy is
authoritative it would fail closed forever — RunKeygen refuses a policy
needing more parties than the committee has, so no custody key could ever
be generated.

Policies now match the validator set each network actually has: 3-of-5 on
mainnet/testnet/devnet, 2-of-3 on localnet. Both keep 2K > N, so neither
admits two disjoint quorums.

Co-authored-by: Hanzo Dev <dev@hanzo.ai>

configs/localnet/mchain.json

@@ -14,7 +14,7 @@
   ],
   "name": "M-Chain",
   "networkId": 1337,
-  "policy": "3-of-5",
+  "policy": "2-of-3",
   "timestamp": 1735689600,
   "version": 1,
   "vm": "mpcvm"

configs/mainnet/mchain.json

@@ -14,7 +14,7 @@
   ],
   "name": "M-Chain",
   "networkId": 1,
-  "policy": "7-of-10",
+  "policy": "3-of-5",
   "timestamp": 1730446786,
   "version": 1,
   "vm": "mpcvm"