v0.2: 8 chain_* read-only Koios passthrough MCP tools

Adds a parallel read-only API surface alongside wallet_*:

  chain_tx_info        full Koios tx_info (any hash)
  chain_address_info   balance + utxos at any address
  chain_pool_list      filter by ticker / pool_id_bech32
  chain_pool_info      detail per pool (delegators, blocks)
  chain_epoch_params   protocol params for an epoch
  chain_asset_info     supply, holders, mint history
  chain_account_info   stake address state
  chain_tip            current chain tip

All passthrough — Koios JSON returned verbatim, no re-shaping.
Network-aware via existing ALDABRA_KOIOS_BASE; mainnet vs preprod
just changes the URL. No keys touched, no signing path. Saves
the bash-curl friction Sulkta flagged 2026-05-05 mid-mainnet
testing arc.

Wire-up: KoiosClient gets `post_raw_json` + `get_raw_json`
helpers that return raw response strings instead of decoding
into typed structures. The chain_* tools are thin wrappers
around those.

ServerInfo `instructions` updated to advertise the chain_*
surface alongside wallet_*.

crates/aldabra-chain/src/koios.rs

@@ -145,6 +145,51 @@ impl KoiosClient {
             .await
             .map_err(|e| ChainError::Decode(e.to_string()))
     }
+
+    /// Generic POST that returns the raw JSON response as a `String` —
+    /// for the `chain_*` MCP passthrough tools where we don't want to
+    /// re-shape Koios's response into typed Rust structures. Caller
+    /// passes a serializable body (often `serde_json::json!({...})`)
+    /// and gets back the response body verbatim.
+    pub async fn post_raw_json<T: Serialize>(
+        &self,
+        path: &str,
+        body: &T,
+    ) -> Result<String, ChainError> {
+        self.http
+            .post(self.url(path))
+            .json(body)
+            .send()
+            .await
+            .map_err(|e| ChainError::Network(e.to_string()))?
+            .error_for_status()
+            .map_err(|e| ChainError::Network(e.to_string()))?
+            .text()
+            .await
+            .map_err(|e| ChainError::Decode(e.to_string()))
+    }
+
+    /// Generic GET (with optional query string) that returns the raw
+    /// JSON response as a `String`. Used for Koios endpoints that
+    /// take filters as query params (`pool_list?ticker=eq.AHL`,
+    /// `epoch_params`, `tip`, etc.).
+    pub async fn get_raw_json(
+        &self,
+        path: &str,
+        query: &[(&str, &str)],
+    ) -> Result<String, ChainError> {
+        self.http
+            .get(self.url(path))
+            .query(query)
+            .send()
+            .await
+            .map_err(|e| ChainError::Network(e.to_string()))?
+            .error_for_status()
+            .map_err(|e| ChainError::Network(e.to_string()))?
+            .text()
+            .await
+            .map_err(|e| ChainError::Decode(e.to_string()))
+    }
 }
 
 fn parse_u64(s: &str, field: &str) -> Result<u64, ChainError> {