json schema polish: count on list, hash alias on verify_certificate (v1)

Acceptance test report flagged three cosmetic JSON-schema gaps:

1. 'list -json' had no top-level 'count' — caller had to use .snapshots|length.
   Added 'count' alongside 'snapshots'.
2. 'verify -json chain' top-level 'kind' was reported null — actually a
   missing field rather than null. Chain results have per-step kind in
   .steps[]; chain-level kind would be misleading. Documented intent
   in README rather than adding the field.
3. MCP 'mithril_verify_certificate' returned 'cert_hash' but agents
   often look for 'hash'. Added 'hash' alias alongside 'cert_hash' in
   both genesis and STM result paths so either lookup works.

End-to-end loop test on mainnet+preprod: full PASS (89-cert mainnet
chain + 90-cert preprod chain both verified, MCP all 8 tools work,
exit codes correct, manifest detection clean). v1-tag-able now.

cmd/mithril-go/main.go

@@ -142,7 +142,11 @@ func cmdList(ctx context.Context, args []string) int {
 		return exitNetwork
 	}
 	if *asJSON {
-		return emitJSON(map[string]any{"network": n.Name, "snapshots": snaps})
+		return emitJSON(map[string]any{
+			"network":   n.Name,
+			"count":     len(snaps),
+			"snapshots": snaps,
+		})
 	}
 	tw := tabwriter.NewWriter(os.Stdout, 0, 0, 2, ' ', 0)
 	fmt.Fprintln(tw, "HASH\tEPOCH\tIMMUTABLE\tSIZE\tCREATED")

cmd/mithril-go/mcp.go

@@ -254,6 +254,7 @@ func registerMCPTools(s *mcp.Server) {
 				verr := verify.GenesisFromJSON(vk, cert.SignedMessage, cert.GenesisSignature, cert.ProtocolMessage)
 				return map[string]any{
 					"kind":      "genesis",
+					"hash":      cert.Hash,
 					"cert_hash": cert.Hash,
 					"epoch":     cert.Epoch,
 					"verified":  verr == nil,
@@ -277,6 +278,7 @@ func registerMCPTools(s *mcp.Server) {
 			verr := stm.Verify([]byte(cert.SignedMessage), ms, avk, params)
 			return map[string]any{
 				"kind":          "stm",
+				"hash":          cert.Hash,
 				"cert_hash":     cert.Hash,
 				"epoch":         cert.Epoch,
 				"signers":       len(ms.Signatures),