cosmos-hub (gaiad): add compose + init.sh + cometbft.Dockerfile

First pure-cosmos chain (validates the §2 cometbft-common.sh lib via real gaiad init).
Single-binary CometBFT client serving :26657 (dshackle chain=cosmos-hub). Statesync
bootstrap (polkachu) since genesis-replay across gaia governance upgrades is impractical.
gzipped genesis. v25.3.2 (live chain version; registry's v27.4.0 not on ghcr). Params
(chain_id/genesis_url/statesync_rpc/seeds/min_gas) from context.yml.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

compose_registry.json

@@ -1690,6 +1690,20 @@
       "core-pigeon-core-pruned-pebble-path"
     ]
   },
+  {
+    "chain": "mainnet",
+    "client": "gaiad",
+    "compose_file": "cosmos/gaiad/cosmos-mainnet-gaiad-pruned",
+    "features": [],
+    "network": "cosmos",
+    "node": null,
+    "relay": null,
+    "stack": null,
+    "type": "pruned",
+    "volumes": [
+      "cosmos-mainnet-gaiad-pruned"
+    ]
+  },
   {
     "chain": "zkevm-mainnet",
     "client": "external-node",